fix(collab): chunked welcome so large session snapshots can join

The host used to ship the entire transcript inside a single welcome
frame, so a multi-MB session spent the guest's 30s first-welcome
timeout on the relay transfer itself: ~1.3 MB took ~3s, ~4.2 MB took
~12s, and ~13.6 MB never arrived before the guest gave up with
'timed out waiting for the host's welcome'.

Bump COLLAB_PROTO to 2 and split the welcome:

- welcome carries metadata only (header, state, agents, entryCount,
  readOnly) and lands in well under one second.
- a train of snapshot-chunk frames (SNAPSHOT_CHUNK_BYTES = 512 KB,
  oversize entries ship alone) carries the transcript. Last chunk
  flips final: true; an empty snapshot still emits one final chunk.
- the host queues welcome + chunks synchronously inside #handleHello,
  preserving the host comment's ordering invariant (later broadcast
  frames cannot interleave between them).
- the TUI guest accumulates chunks under a SNAPSHOT_PROGRESS_TIMEOUT_MS
  that resets per chunk; only after final does it write the replica
  jsonl, switchSession, and render. The first-welcome timeout still
  guards arrival of the small welcome.
- the collab-web GuestClient streams entries into the snapshot as
  chunks arrive and flips phase to 'live' on final.

Includes a contract test (in-process relay) asserting the welcome is
metadata-only, the chunk train fans the 1.5 MB synthetic transcript
across multiple frames with only the last marked final, and the
flattened entries match the source snapshot.

Fixes #3144

packages/wire/test/constants.test.ts

@@ -9,7 +9,7 @@ import {
 
 describe("collab wire constants", () => {
 	it("exports the protocol constants consumed by host, guest, and relay links", () => {
-		expect(COLLAB_PROTO).toBe(1);
+		expect(COLLAB_PROTO).toBe(2);
 		expect(COLLAB_PROMPT_MESSAGE_TYPE).toBe("collab-prompt");
 		expect(ENVELOPE_HEADER_LENGTH).toBe(4);
 		expect(ROOM_ID_BYTES).toBe(16);