feat: WebSocket flush protocol for force-save before Preview/Publish

[kptdobe]

Summary

• Adds messageFlushRequest (type 2) and messageFlushResponse (type 3) constants to the WebSocket protocol so the DA editor client can request an immediate force-save before Preview or Publish reads the document from da-admin.
• Implements the flush case in messageListener: awaits doc.flushSave() then sends a binary ack with ok=1 on success or ok=0 plus an error string on failure.
• Introduces a savingPromise variable alongside the existing saving flag so that flushSave can wait for any already-in-flight PUT to complete before resolving — this prevents the ack being sent before the save actually finishes.

Problem

When Preview or Publish is clicked, the client sends a flush request over the existing WebSocket connection. Without this server-side handler, the da-collab Durable Object has no way to know it should flush its debounced 2-second save early. The race is subtle: if a PUT is already in progress when flushSave is called, the previous code would immediately resolve, causing an ack before the in-flight save had finished.

Protocol

varuint	meaning
2	messageFlushRequest (client → server)
3	messageFlushResponse (server → client): followed by ok (1=success, 0=failure) and, on failure, an error string

Test plan

• messageFlushRequest sends flush response ack with ok=1 after flushSave — happy path
• messageFlushRequest sends flush response with ok=0 when flushSave throws — error path encodes error message
• messageFlushRequest works when doc has no flushSave (still sends ok ack) — graceful no-op
• Flush waits for an in-flight save before resolving — savingPromise race-condition fix
• All 117 existing tests continue to pass with 100% statement/function coverage

🤖 Generated with Claude Code

[kptdobe]

Branch rebuilt and force-pushed to isolate changes.

This PR (feat/flush-protocol) now contains only the flush-protocol changes:

• export const messageFlushRequest = 2 and export const messageFlushResponse = 3 constants
• savingPromise tracking in bindState so in-flight saves can be awaited
• ydoc.flushSave waits for any in-flight savingPromise before starting a new save
• ydoc.cancelSave helper
• messageListener made async
• messageFlushRequest case in the switch: awaits flushSave, sends a messageFlushResponse ack with ok=1 or ok=0+error string

Removed from this PR: all lastsync CF storage marker changes (those belong to #153).

Tests: 115 passing, 100% coverage. Lint clean.

[adobe-bot]

🎉 This PR is included in version 1.5.0 🎉

The release is available on:

• v1.5.0
• GitHub release

Your semantic-release bot 📦🚀