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.

2 files changed, 2 insertions, 4 deletions

diff --git a/sl0ppij/zellij-utils/src/plugin_api/event.rs b/sl0ppij/zellij-utils/src/plugin_api/event.rs
index 87af043..7bb1c7a 100644
--- a/sl0ppij/zellij-utils/src/plugin_api/event.rs
+++ b/sl0ppij/zellij-utils/src/plugin_api/event.rs
@@ -2495,8 +2495,7 @@ fn serialize_mode_update_event_with_non_default_values() {
             pane_title_alignment: Default::default(),
             pane_gap_x: Default::default(),
             pane_gap_y: Default::default(),
-            pane_padding_x: Default::default(),
-            pane_padding_y: Default::default(),
+            pane_padding: Default::default(),
         },
         capabilities: PluginCapabilities { arrow_fonts: false },
         session_name: Some("my awesome test session".to_owned()),
diff --git a/sl0ppij/zellij-utils/src/plugin_api/style.rs b/sl0ppij/zellij-utils/src/plugin_api/style.rs
index 266ab7f..527f984 100644
--- a/sl0ppij/zellij-utils/src/plugin_api/style.rs
+++ b/sl0ppij/zellij-utils/src/plugin_api/style.rs
@@ -28,8 +28,7 @@ impl TryFrom<ProtobufStyle> for Style {
             pane_title_alignment: Default::default(),
             pane_gap_x: Default::default(),
             pane_gap_y: Default::default(),
-            pane_padding_x: Default::default(),
-            pane_padding_y: Default::default(),
+            pane_padding: Default::default(),
         })
     }
 }