feat: add per-pane default foreground/background color support (#4737)

* feat: add per-pane default foreground/background color support

Add per-pane default foreground and background colors, configurable
through four surfaces:

- OSC 10/11 escape sequences (set/query) and OSC 110/111 (reset)
- CLI action: zellij action set-pane-color --pane-id <id> --fg/--bg
- Layout config: default_fg/default_bg properties on pane nodes
- Plugin API: set_pane_color() SDK function and PaneInfo read-side

Colors are applied at render time as a fallback when cells have no
explicit foreground/background, without mutating stored grid data.

Closes #4712

* refactor: address review feedback for pane color support

- Extract adjust_styles_for_custom_bg_fg in output/mod.rs
- Move color parsing (xparse_color) from tab into Grid
- Move color serialization (ansi_code_to_color_string) into Grid
- Pane trait now uses Option<String> for color getters/setters
- Add colors to session serialization (dump-layout)
- Add default_fg/default_bg to protobuf IPC schema to fix
  layout-in-session color loss
- Update example layout with tab-bar and status-bar

* fix stack overflow and ordering

* rustfmt

* update e2e snapshots

---------

Co-authored-by: Karl Bunch <karl@karlbunch.com>
Co-authored-by: Aram Drevekenin <aram@poor.dev>

1 files changed, 13 insertions, 0 deletions

diff --git a/example/layouts/pane_colors_layout.kdl b/example/layouts/pane_colors_layout.kdl
new file mode 100644
index 00000000..ad5b3705
--- /dev/null
+++ b/example/layouts/pane_colors_layout.kdl
@@ -0,0 +1,13 @@
+layout {
+    pane size=1 borderless=true {
+        plugin location="tab-bar"
+    }
+    pane split_direction="Vertical" {
+        pane name="dark-blue-bg" default_bg="#001a3a"
+        pane name="green-on-black" default_fg="#00e000" default_bg="#0a0a0a"
+    }
+    pane name="default-colors"
+    pane size=1 borderless=true {
+        plugin location="status-bar"
+    }
+}