sl0p: CSS-style 1/2/4-value shorthand for inner padding

`padding` now accepts:
  padding 1          -> all four sides
  padding 1 2        -> up/down = 1, left/right = 2
  padding 1 2 3 4    -> up=1 down=2 left=3 right=4

NOTE the 4-value order is UP DOWN LEFT RIGHT as requested, deliberately NOT
CSS's clockwise top-right-bottom-left. Documented on PanePadding and at the
parser, since the similarity to CSS invites the wrong assumption.

3 or 5 values are rejected with a message naming the accepted forms, rather
than being silently reinterpreted. Negative values clamp to 0.

ASPECT: only the 1-value form is aspect-corrected (left/right multiplied by
gap_aspect) so `padding 1` looks square on a non-square cell grid. The 2- and
4-value forms are literal: specifying axes separately means the caller is
already controlling horizontal size, and silently doubling it would surprise.

Style now carries a PanePadding {top,bottom,left,right} instead of an x/y
pair, which also collapses the reconfigure plumbing back to a single field
rather than growing another parameter per side.

to_kdl always emits the explicit 4-value form: emitting the 1-value form
would re-apply gap_aspect on the next read and silently double the padding
on every config round-trip.

Baselines held: zellij-utils 435/5, zellij-server 1421/30.

7 files changed, 33 insertions, 51 deletions

diff --git a/zellij-server/src/lib.rs b/zellij-server/src/lib.rs
index a80bda2b..fe5b3b19 100644
--- a/zellij-server/src/lib.rs
+++ b/zellij-server/src/lib.rs
@@ -462,8 +462,7 @@ impl SessionMetaData {
                     pane_title_alignment: new_config.ui.pane_frames.title_alignment,
                     pane_gap_x: new_config.ui.pane_frames.resolved_gap().0,
                     pane_gap_y: new_config.ui.pane_frames.resolved_gap().1,
-                    pane_padding_x: new_config.ui.pane_frames.resolved_padding().0,
-                    pane_padding_y: new_config.ui.pane_frames.resolved_padding().1,
+                    pane_padding: new_config.ui.pane_frames.resolved_padding(),
                     stacked_resize: new_config.options.stacked_resize.unwrap_or(true),
                     stacked_pane_list: new_config.options.stacked_pane_list.unwrap_or(true),
                     default_editor: new_config.options.scrollback_editor.clone(),
@@ -1008,8 +1007,7 @@ pub fn start_server_impl(
                         pane_title_alignment: config.ui.pane_frames.title_alignment,
                         pane_gap_x: config.ui.pane_frames.resolved_gap().0,
                         pane_gap_y: config.ui.pane_frames.resolved_gap().1,
-                        pane_padding_x: config.ui.pane_frames.resolved_padding().0,
-                        pane_padding_y: config.ui.pane_frames.resolved_padding().1,
+                        pane_padding: config.ui.pane_frames.resolved_padding(),
                     },
                 };
 
@@ -1196,8 +1194,7 @@ pub fn start_server_impl(
                         pane_title_alignment: config.ui.pane_frames.title_alignment,
                         pane_gap_x: config.ui.pane_frames.resolved_gap().0,
                         pane_gap_y: config.ui.pane_frames.resolved_gap().1,
-                        pane_padding_x: config.ui.pane_frames.resolved_padding().0,
-                        pane_padding_y: config.ui.pane_frames.resolved_padding().1,
+                        pane_padding: config.ui.pane_frames.resolved_padding(),
                     },
                 };
 
diff --git a/zellij-server/src/panes/floating_panes/mod.rs b/zellij-server/src/panes/floating_panes/mod.rs
index 8252612c..3e5e80f8 100644
--- a/zellij-server/src/panes/floating_panes/mod.rs
+++ b/zellij-server/src/panes/floating_panes/mod.rs
@@ -1725,16 +1725,14 @@ impl FloatingPanes {
         title_alignment: zellij_utils::data::PaneTitleAlignment,
         gap_x: usize,
         gap_y: usize,
-        padding_x: usize,
-        padding_y: usize,
+        padding: zellij_utils::data::PanePadding,
     ) {
         self.style.pane_title_alignment = title_alignment;
         self.style.pane_gap_x = gap_x;
         self.style.pane_gap_y = gap_y;
-        self.style.pane_padding_x = padding_x;
-        self.style.pane_padding_y = padding_y;
+        self.style.pane_padding = padding;
         for pane in self.panes.values_mut() {
-            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding_x, padding_y);
+            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding);
         }
         // Unlike the tiled panes, `set_pane_frame_style` only re-runs the
         // content-offset sweep for a fullscreen floating pane, so gap/padding
diff --git a/zellij-server/src/panes/plugin_pane.rs b/zellij-server/src/panes/plugin_pane.rs
index 4f4e138d..6c1e8707 100644
--- a/zellij-server/src/panes/plugin_pane.rs
+++ b/zellij-server/src/panes/plugin_pane.rs
@@ -773,10 +773,10 @@ impl Pane for PluginPane {
             Offset::default()
         } else {
             Offset {
-                top: self.gap_offset.top.saturating_add(self.style.pane_padding_y),
-                bottom: self.gap_offset.bottom.saturating_add(self.style.pane_padding_y),
-                left: self.gap_offset.left.saturating_add(self.style.pane_padding_x),
-                right: self.gap_offset.right.saturating_add(self.style.pane_padding_x),
+                top: self.gap_offset.top.saturating_add(self.style.pane_padding.top),
+                bottom: self.gap_offset.bottom.saturating_add(self.style.pane_padding.bottom),
+                left: self.gap_offset.left.saturating_add(self.style.pane_padding.left),
+                right: self.gap_offset.right.saturating_add(self.style.pane_padding.right),
             }
         };
         self.content_offset =
@@ -915,14 +915,12 @@ impl Pane for PluginPane {
         title_alignment: zellij_utils::data::PaneTitleAlignment,
         gap_x: usize,
         gap_y: usize,
-        padding_x: usize,
-        padding_y: usize,
+        padding: zellij_utils::data::PanePadding,
     ) {
         self.style.pane_title_alignment = title_alignment;
         self.style.pane_gap_x = gap_x;
         self.style.pane_gap_y = gap_y;
-        self.style.pane_padding_x = padding_x;
-        self.style.pane_padding_y = padding_y;
+        self.style.pane_padding = padding;
         self.frame.clear();
     }
     fn set_should_be_suppressed(&mut self, should_be_suppressed: bool) {
diff --git a/zellij-server/src/panes/terminal_pane.rs b/zellij-server/src/panes/terminal_pane.rs
index 8c44d7b1..7fb78171 100644
--- a/zellij-server/src/panes/terminal_pane.rs
+++ b/zellij-server/src/panes/terminal_pane.rs
@@ -991,10 +991,10 @@ impl Pane for TerminalPane {
             Offset::default()
         } else {
             Offset {
-                top: self.gap_offset.top.saturating_add(self.style.pane_padding_y),
-                bottom: self.gap_offset.bottom.saturating_add(self.style.pane_padding_y),
-                left: self.gap_offset.left.saturating_add(self.style.pane_padding_x),
-                right: self.gap_offset.right.saturating_add(self.style.pane_padding_x),
+                top: self.gap_offset.top.saturating_add(self.style.pane_padding.top),
+                bottom: self.gap_offset.bottom.saturating_add(self.style.pane_padding.bottom),
+                left: self.gap_offset.left.saturating_add(self.style.pane_padding.left),
+                right: self.gap_offset.right.saturating_add(self.style.pane_padding.right),
             }
         };
         self.content_offset =
@@ -1269,14 +1269,12 @@ impl Pane for TerminalPane {
         title_alignment: zellij_utils::data::PaneTitleAlignment,
         gap_x: usize,
         gap_y: usize,
-        padding_x: usize,
-        padding_y: usize,
+        padding: zellij_utils::data::PanePadding,
     ) {
         self.style.pane_title_alignment = title_alignment;
         self.style.pane_gap_x = gap_x;
         self.style.pane_gap_y = gap_y;
-        self.style.pane_padding_x = padding_x;
-        self.style.pane_padding_y = padding_y;
+        self.style.pane_padding = padding;
         self.frame.clear();
     }
     fn drain_fake_cursors(&mut self) -> Option<HashSet<(usize, usize)>> {
diff --git a/zellij-server/src/panes/tiled_panes/mod.rs b/zellij-server/src/panes/tiled_panes/mod.rs
index 835c8e74..4d8f0a4a 100644
--- a/zellij-server/src/panes/tiled_panes/mod.rs
+++ b/zellij-server/src/panes/tiled_panes/mod.rs
@@ -3156,16 +3156,14 @@ impl TiledPanes {
         title_alignment: zellij_utils::data::PaneTitleAlignment,
         gap_x: usize,
         gap_y: usize,
-        padding_x: usize,
-        padding_y: usize,
+        padding: zellij_utils::data::PanePadding,
     ) {
         self.style.pane_title_alignment = title_alignment;
         self.style.pane_gap_x = gap_x;
         self.style.pane_gap_y = gap_y;
-        self.style.pane_padding_x = padding_x;
-        self.style.pane_padding_y = padding_y;
+        self.style.pane_padding = padding;
         for pane in self.panes.values_mut() {
-            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding_x, padding_y);
+            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding);
         }
     }
     pub fn stack_panes(
diff --git a/zellij-server/src/screen.rs b/zellij-server/src/screen.rs
index c3c06ca9..438fee2d 100644
--- a/zellij-server/src/screen.rs
+++ b/zellij-server/src/screen.rs
@@ -43,7 +43,7 @@ use zellij_utils::data::{
     CommandOrPlugin, Direction, EventType, FloatingPaneCoordinates, GetFocusedPaneInfoResponse,
     HostTerminalThemeMode, KeyWithModifier, LayoutInfo, LayoutWithError, ListPanesResponse,
     ListTabsResponse, NewPanePlacement, PaneContents, PaneInfo, PaneListEntry, PaneManifest,
-    PaneRenderReport, PaneScrollbackResponse, PaneTitleAlignment, PluginPermission,
+    PaneRenderReport, PanePadding, PaneScrollbackResponse, PaneTitleAlignment, PluginPermission,
     RegexHighlight, Resize,
     ResizeStrategy, SessionInfo, Styling, TabInfo, ThemeHue, WebSharing,
 };
@@ -779,8 +779,7 @@ pub enum ScreenInstruction {
         pane_title_alignment: PaneTitleAlignment,
         pane_gap_x: usize,
         pane_gap_y: usize,
-        pane_padding_x: usize,
-        pane_padding_y: usize,
+        pane_padding: PanePadding,
         stacked_resize: bool,
         stacked_pane_list: bool,
         default_editor: Option<PathBuf>,
@@ -6270,8 +6269,7 @@ impl Screen {
         pane_title_alignment: PaneTitleAlignment,
         pane_gap_x: usize,
         pane_gap_y: usize,
-        pane_padding_x: usize,
-        pane_padding_y: usize,
+        pane_padding: PanePadding,
         stacked_resize: bool,
         stacked_pane_list: bool,
         default_editor: Option<PathBuf>,
@@ -6331,7 +6329,7 @@ impl Screen {
             tab.update_copy_options(&self.copy_options);
             // must precede set_pane_frames: gap/padding are baked into
             // content_offset by the sweep that set_pane_frames triggers
-            tab.update_frame_layout(pane_title_alignment, pane_gap_x, pane_gap_y, pane_padding_x, pane_padding_y);
+            tab.update_frame_layout(pane_title_alignment, pane_gap_x, pane_gap_y, pane_padding);
             tab.set_pane_frames(pane_frame_style);
             tab.update_arrow_fonts(should_support_arrow_fonts);
             tab.update_advanced_mouse_actions(advanced_mouse_actions);
@@ -10850,8 +10848,7 @@ pub(crate) fn screen_thread_main(
                 pane_title_alignment,
                 pane_gap_x,
                 pane_gap_y,
-                pane_padding_x,
-                pane_padding_y,
+                pane_padding,
                 stacked_resize,
                 stacked_pane_list,
                 default_editor,
@@ -10882,8 +10879,7 @@ pub(crate) fn screen_thread_main(
                         pane_title_alignment,
                         pane_gap_x,
                         pane_gap_y,
-                        pane_padding_x,
-                        pane_padding_y,
+                        pane_padding,
                         stacked_resize,
                         stacked_pane_list,
                         default_editor,
diff --git a/zellij-server/src/tab/mod.rs b/zellij-server/src/tab/mod.rs
index c9047d68..ec088422 100644
--- a/zellij-server/src/tab/mod.rs
+++ b/zellij-server/src/tab/mod.rs
@@ -751,8 +751,7 @@ pub trait Pane {
         _title_alignment: zellij_utils::data::PaneTitleAlignment,
         _gap_x: usize,
         _gap_y: usize,
-        _padding_x: usize,
-        _padding_y: usize,
+        _padding: zellij_utils::data::PanePadding,
     ) {
     }
     fn set_should_be_suppressed(&mut self, _should_be_suppressed: bool) {}
@@ -7420,20 +7419,18 @@ impl Tab {
         title_alignment: zellij_utils::data::PaneTitleAlignment,
         gap_x: usize,
         gap_y: usize,
-        padding_x: usize,
-        padding_y: usize,
+        padding: zellij_utils::data::PanePadding,
     ) {
         self.style.pane_title_alignment = title_alignment;
         self.style.pane_gap_x = gap_x;
         self.style.pane_gap_y = gap_y;
-        self.style.pane_padding_x = padding_x;
-        self.style.pane_padding_y = padding_y;
+        self.style.pane_padding = padding;
         self.floating_panes
-            .update_pane_frame_layout(title_alignment, gap_x, gap_y, padding_x, padding_y);
+            .update_pane_frame_layout(title_alignment, gap_x, gap_y, padding);
         self.tiled_panes
-            .update_pane_frame_layout(title_alignment, gap_x, gap_y, padding_x, padding_y);
+            .update_pane_frame_layout(title_alignment, gap_x, gap_y, padding);
         for (_, pane) in self.suppressed_panes.values_mut() {
-            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding_x, padding_y);
+            pane.update_frame_layout(title_alignment, gap_x, gap_y, padding);
         }
     }
     pub fn update_arrow_fonts(&mut self, should_support_arrow_fonts: bool) {