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/Cargo.toml

@@ -25,6 +25,9 @@ egui_dock.workspace = true
 # Editor preferences file I/O reads/writes the same RON shape `Settings`
 # exports; the engine already pulls `ron` in, the editor now does too.
 ron.workspace = true
+# Editor-owned settings sections (e.g. the External Editor preference) derive
+# their own Serialize/Deserialize for the Settings store.
+serde.workspace = true
 
 # PTY-backed terminal panel (Stage 10): run interactive/TUI programs — shells,
 # REPLs, and AI-agent CLIs like `claude` — inside the editor. `portable-pty`