M9: collaboration: per-user undo, rejections, reconnect (#105)

* M9: collaboration: per-user undo, rejections, reconnect

Step 3 of docs/collab-protocol.md, the parts that don't need the transport:

- Per-user undo and redo (figcraft_collab::Client::{undo, redo}): each undoable batch is a step
  (values before/after by target, nodes created/deleted). Others' batches ordered after the step
  take its targets for good; creates invert to deletes (not when others built inside), deletes
  re-create the subtree. An undo or redo waits for every earlier batch and is remade from the
  confirmed state when it goes out; derived data follows as a follow-up batch.
- Rejections: refused batches leave the history (with an undo waiting on them) and queue a
  Notice; the engine shows it through Session::message (the UI's toast).
- Offline queue and reconnect: Hello.resume {session, client_id, seq}; the server keeps a
  bounded log and answers with the missed batches or a snapshot, plus what it did with the
  client's batches (processed {last_batch, refused}); queued batches then go out.
  Server::state / restore persist the session (client-id counter, log, session id).
- Presence carries editing_text {node, caret, selection}.
- Engine: documents in a live session (LiveDoc, Session::collab_*): commands, gestures and
  groups sync as one step each; edit.undo / edit.redo route to the per-user history; remote
  edits show under a gesture in progress. Outside a session snapshot undo is unchanged.
- Sim: dropped connections, undo/redo, and a checker that no undo reverts a change by someone
  else its author had seen. New tests in undo_tests, reconnect_tests, collab_live_tests,
  collab_undo_tests.
- CLAUDE.md / AGENTS.md: collab in the L2 layer list.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M9: collaboration wording in the progress docs after per-user undo and reconnect

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M9: protocol doc: what the engine's live-session tests cover

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -482,7 +482,9 @@ and a file keeps the newest 20 autosaves (user versions are never dropped for th
 `collab.diff {against? (undo: the last undoable action, the default | saved: since the last
 save)}` returns `{changes, count, bytes}`: the change batch a live session would send for that edit
 (per-property `create` / `set` / `delete` and document-level changes, see
-`docs/collab-protocol.md`) and its binary size. It changes nothing.
+`docs/collab-protocol.md`) and its binary size. It changes nothing. In a live session, where undo
+is per user (`edit.undo` / `edit.redo` take back only your own steps, see
+`docs/collab-protocol.md`), only `saved` applies.
 
 ### Live sessions
 
@@ -502,11 +504,16 @@ what happened since the last one and sends your edits (a joined file opens as a
 - `collab.peers {}` → `{available, status (off|connecting|live|reconnecting|failed), reason?,
   attempt?, role (host|guest), me, name, color, fileName, pages, link, url, address, network,
   follow, syncing (the document arrived), document (the index of the synced open document), peers: [{id, name, color, presence: {cursor: [page, x, y], selection: [ids], viewport:
-  [page, x, y, w, h], editing_text}}]}`.
+  [page, x, y, w, h], editing_text: {node, caret, selection?}}}]}`.
 - `collab.cursor {x, y}` puts your cursor at a page position (omit both to hide it): an agent's
   pointer. Your selection, page and view go out by themselves when they change.
 - `collab.follow {client}` follows that person's viewport; `collab.follow {}` stops (so does any
   pan or zoom of your own, or Escape).
+- On the session's document every command (or `batch`, or gesture) is one step of your own undo
+  history: `edit.undo` / `edit.redo` take back only your steps and leave what others changed since.
+  An edit the host refuses (someone deleted the layer) is rolled back and the reason is the
+  session's message. While the connection is reconnecting, editing goes on; the edits go out when
+  it is back (the same client, catching up with what it missed).
 
 ### Plugins