Editor: open scripts in an external editor

Stage-10 editor-UX follow-up, second half of in-editor script authoring:

- "✏ Edit" button on the Script inspector section (enabled once a
  source is assigned) and double-click on any script in the Project
  panel open the .rhai in the user's editor. Non-script assets
  double-click through xdg-open.
- Resolution order: the new External Editor preference command (spawned
  detached as `<command> <file>`, flags allowed), else $VISUAL/$EDITOR
  in a new Terminal-panel tab (TUI editors work in-editor via the
  existing PTY widget), else xdg-open.
- New `editor.external_editor` settings section (ExternalEditorPrefs)
  registered by EditorState, persisted with the preferences file, with
  a rich row in the Preferences window; registration covered by a unit
  test. Editor crate now depends on serde directly for its own
  sections.

Saved edits flow back through the file watcher's live reload, including
into a playing scene. GUI piece — needs an eye-check before promotion.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

editor/src/state.rs

@@ -209,6 +209,20 @@ impl Default for UiDoc {
     }
 }
 
+/// The settings section holding [`ExternalEditorPrefs`].
+pub const EXTERNAL_EDITOR_SECTION: &str = "editor.external_editor";
+
+/// Preferences for opening a script (or other text asset) in an editor —
+/// registered as the [`EXTERNAL_EDITOR_SECTION`] settings section, editable in
+/// Preferences, persisted to `~/.config/oxide/editor.ron` like the bindings.
+#[derive(Debug, Clone, Default, serde::Serialize, serde::Deserialize)]
+pub struct ExternalEditorPrefs {
+    /// Command to launch, invoked as `<command> <file>` (whitespace-split; may
+    /// carry its own flags, e.g. `"code -g"`). **Empty (the default) = auto**:
+    /// run `$VISUAL`/`$EDITOR` in an editor Terminal tab, else `xdg-open`.
+    pub command: String,
+}
+
 impl EditorState {
     /// A blank state with an empty scene, no open project, and the editor's
     /// default action bindings registered (`F` toggle, WASD/QE move, Shift
@@ -224,6 +238,7 @@ impl EditorState {
         bindings::register_defaults(&mut actions);
         let mut settings = Settings::new();
         settings.register::<ActionOverrides>(bindings::SETTINGS_SECTION);
+        settings.register::<ExternalEditorPrefs>(EXTERNAL_EDITOR_SECTION);
         let mut registry = TypeRegistry::new();
         register_builtin_types(&mut registry);
         // Seed a small, generally-useful set of named layers (besides the
@@ -590,4 +605,17 @@ mod tests {
             .get::<oxide_physics::RigidBody>(restored)
             .is_some());
     }
+
+    #[test]
+    fn external_editor_section_is_registered_and_defaults_to_auto() {
+        let state = EditorState::new();
+        let prefs = state
+            .settings
+            .get::<ExternalEditorPrefs>(EXTERNAL_EDITOR_SECTION)
+            .expect("external-editor settings section must be registered");
+        assert!(
+            prefs.command.is_empty(),
+            "default is empty = auto ($VISUAL/$EDITOR terminal tab, else xdg-open)"
+        );
+    }
 }