Git snapshot + coordinator - #244

Prevent concurrent callers from creating divergent checkout IDs or exposing a partially written canonical ID. Use a dedicated slow-path advisory lock with a second read, then persist through a synced unique temporary file and atomic rename; add coverage for convergence and interruption cases. Document the locking behavior and the repository's safe filesystem-test pattern.

Plan: mutation-cursor-runtime-coordinator (T01)

Co-authored-by: SCE <sce@crocoder.dev>

Serialize the mutation-cursor runtime's future worktree critical section without allowing a stuck holder to block later invocations indefinitely. Add private runtime scaffolding and a WorktreeLock that polls File::try_lock() against a caller-supplied timeout, reports a matchable timeout error, and releases ownership through RAII. The lock behavior is covered for contention, timeout, independent worktrees, and stale lock files.

Plan: mutation-cursor-runtime-coordinator (T02)

Co-authored-by: SCE <sce@crocoder.dev>

Capture complete worktree state through a temporary index without altering the real index or working tree. Store snapshot objects in Git's normal object database, pin durable trees with SCE-owned refs, and expose tree diffs for the future coordinator; add focused coverage for state capture, pruning, unborn HEADs, and SHA-256 repositories.

Update mutation-trace architecture documentation and record the completed implementation and verification details.
Plan: mutation-cursor-runtime-coordinator (T03)

Co-authored-by: SCE <sce@crocoder.dev>

The mutation-cursor runtime needs an imperative integration layer that turns Git observations and boundary events into durable protocol state without duplicating evidence or losing concurrent updates.

Add the generic coordinator pipeline with snapshot capture and pinning, idempotent worktree and scope materialization, recovery handling, bounded CAS reload/recompute retries, snapshot-failure taint persistence, and focused concurrency and lifecycle tests. Synchronize the mutation-trace context documents and mark the completed plan task.

The public lock-wrapped coordinate() entrypoint remains follow-up work; this change keeps the internal pipeline deterministic through the SnapshotCapture seam.

Plan: mutation-cursor-runtime-coordinator (T04)

Co-authored-by: SCE <sce@crocoder.dev>

Expose coordinate() as the runtime boundary for driving mutation-cursor processing against a real worktree while preventing concurrent same-worktree observations from racing.

Resolve the Git directory, acquire a bounded WorktreeLock, derive WorktreeId from checkout identity, and delegate to the existing snapshot/protocol/CAS pipeline; add serialization coverage and update the runtime documentation and plan.

Plan: mutation-cursor-runtime-coordinator (T05)

Co-authored-by: SCE <sce@crocoder.dev>

Add end-to-end coverage for linked-worktree coordination, checkout identity convergence, and snapshot failure recovery through coordinate(). Record completion of mutation-cursor-runtime-coordinator T06 and the resulting runtime behavior.

Co-authored-by: SCE <sce@crocoder.dev>

Mark the mutation-cursor runtime coordinator plan complete after the coordinator, snapshot, locking, CAS-retry, taint-recovery, and checkout-ID behaviors passed the recorded validation suite. Preserve the validation evidence and document the intentionally deferred ref reconciliation and external-taint work.

Plan: mutation-cursor-runtime-coordinator
Tasks: AC1-AC13

Co-authored-by: SCE <sce@crocoder.dev>