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>

AGENTS.md

@@ -75,7 +75,7 @@ code.
   incumbent's menu catalog in `crates/engine/src/catalog.rs` (the parity metric: `cargo xtask parity`).
 - **Programmatic calls never open dialogs.** Commands only *request* UI (`Session::request_ui`); the
   control channel drops dialog requests.
-- **Layering** is enforced by `cargo xtask layers`: geom/color (L0) → doc (L1) → layout/text/pathops/plugin
+- **Layering** is enforced by `cargo xtask layers`: geom/color (L0) → doc (L1) → layout/text/pathops/plugin/collab
   (L2) → render/format (L3) → svg/pdf (L4) → engine (L5) → ui-egui/mcp (L6) → apps. Nothing below
   L6 depends on egui/eframe/rfd/winit. **The UI crate is swappable.**
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /