Support checkpoint request protocol

[simolus3]

This adds support for checkpoint requests, enabled as an option:

1. Use new /sync/checkpoint-request request instead of /write-checkpoint2.json.
2. Adopt checkpoint state, seed with connect request from core extension.
3. Repost checkpoint requests until they're included in a completed checkpoint on the sync stream.

Tests are ported from prior JS, Dart, Kotlin and DotNet PRs adding the same thing.

AI use: I used Claude Code to review this and to help port tests.