M4.12: docs: ROADMAP, parity, web, preferences and agents brought up to date after M3 and M4

- parity: the status note covers the M3.9–M4.12 passes (feature counts unchanged: they fixed
  behaviour inside features already counted done) and the GPU effect count matches EFF-5 (280);
  the Layers, Compositions, Interface, Project and Preview rows name what is left and what
  landed; landed items in "Highest-value gaps" are struck through with where they landed, the
  gaps the passes left are items 16–18, and the closing paragraph no longer says 69 menu entries
  are disabled.
- ROADMAP: what is left includes the preview gaps.
- web: a restored session with unsaved changes stays modified and asks before closing.
- preferences: Adaptive Resolution Limit applies while dragging, not to playing previews.
- agents: nested comp settings and New Comp from Selection for several items.

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

docs/web.md

@@ -90,7 +90,8 @@ writes are atomic: a swap file replaced on close). Keys:
 | `files/<path>` | the file table: imported media, projects saved with File ▸ Save / Save As or `saveToBrowser`, auto-saves |
 
 On the next visit the app reopens the snapshot (unsaved changes stay unsaved: the project is
-dirty), registers every stored media file with the media pool so its footage decodes, and File ▸
+modified until saved, `Session::mark_unsaved`, so File ▸ New, Open, Close Project and Revert still
+ask to save it first), registers every stored media file with the media pool so its footage decodes, and File ▸
 Open Recent reopens saved projects from storage. Without a snapshot (first visit) it opens the
 demo project. Render outputs are not stored: they download.