M3.9: closing a modified project asks to save it first; the modified mark follows undo

- Quit and the window's close button, File ▸ New Project, Open Project, Open Recent, Close
  Project, the demo project and a Home template show Save / Don't Save / Cancel when the
  project has unsaved changes (Save asks for a path when untitled; cancelling that keeps the
  project open). File ▸ Revert asks to discard. Before, all of them dropped unsaved work
  without asking.
- The window title shows the project's path and `*` while it is modified.
- The modified mark follows undo: the session keeps the snapshot it last opened or saved
  (`Session::saved_project`), so undoing, jumping in the History panel or rolling back a batch
  to the saved state clears it.
- An edit that changes nothing (a value set to what it already is) records no undo step and
  doesn't mark the project modified (`Project::same_content`, comparing untouched comps by
  pointer).
- Agents' `engine.execute` / MCP calls are never asked; `ui.menu.invoke` shows the prompt like a
  click (automation ids `dialog.unsaved.save` / `dontSave` / `revert` / `cancel`), and the
  control channel's `app.quit {force: true}` skips it.
- Docs: architecture, agents, control protocol, parity.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -531,7 +531,16 @@ What keeps it fast:
   app; inline on wasm32).
 - **Undo is a pointer swap.** History stores `Arc<Project>` snapshots and comps are `Arc`s, so
   undo and redo of any edit are constant time. An edit copies the comps it changes (a one-layer
-  edit in a 5,000-layer comp copies that comp: ~8 ms).
+  edit in a 5,000-layer comp copies that comp: ~8 ms). An edit that changes nothing (a value
+  set to what it already was) records no step: `Project::same_content` compares the comps the
+  edit left shared by pointer, so the check costs about as much as the copy.
+- **Modified follows undo.** The session keeps the snapshot it last opened or saved
+  (`Session::saved_project`); the project is unmodified whenever the current snapshot is that
+  one, so undoing (or jumping in the History panel, or rolling back a batch) to the saved state
+  clears the modified mark. The window title shows the project and `*` while it is modified, and
+  closing a modified project (Quit, the window's close button, New Project, Open, Open Recent,
+  Close Project, Revert, the demo project, a Home template) asks Save / Don't Save / Cancel first
+  (`ui-egui/src/panels/unsaved.rs`). Agents' `engine.execute` calls are never asked.
 
 Numbers (Apple Silicon, 14 cores, release build, best of 3–20 runs; the machine was shared with
 other builds, load average 150–220, so treat them as orders of magnitude). "Before" is the