fix(webview): coordinate durable viewStates writes across extension hosts (F2 follow-up) #2314

Summary

Coordinate viewStates read-modify-write operations across independent extension hosts. This is the explicit F2 follow-up deferred from PR #1921, under parent #1536.

ClineProvider.persistedViewStateWriteQueue is static within one extension-host process. Separate VS Code windows have separate queues but share extension global state. Two hosts can read the same map, update different entries, and write competing whole-map snapshots. The later write can remove the other host's mode or profile pin.

Required changes

• Provide a cross-host coordination or conflict-safe merge mechanism for the durable viewStates store.
• Read authoritative state inside that mechanism before applying and committing each mutation. A lock around a stale ContextProxy or globalState cached read is not sufficient.
• Evaluate the existing guarded-write/merge abstraction used by safeWriteJson. Reuse the established file-safety layer where compatible, or document why another storage/coordination design is necessary.
• Route saves, clears/reset, temporary-to-stable re-keying, pruning, profile re-pointing, and rollback through the same coordinated persistence boundary so no whole-map writer bypasses it.
• Preserve existing per-view isolation and persistence-failure behavior. Do not broaden this unit into unrelated profile-routing reconciliation between PR test(webview): F2a - profile-mutation state semantics and the mode-rollback guard #1919 and PR test(webview): ClineProvider parallelMode suite with viewStates pruning edges #1921.

Affected areas

• src/core/webview/ClineProvider.ts: persistedViewStateWriteQueue, getPersistedViewStates, writePersistedViewStates, and all durable viewStates mutations.
• src/core/config/ContextProxy.ts: global-state cache and persistence integration.
• The existing safeWriteJson guarded-write/merge layer and focused cross-host regression coverage.

Acceptance criteria

• A deterministic regression uses two independent extension hosts or processes with independently stale caches against one durable store.
• Overlapping writes to different view IDs retain both entries and their mode/profile selections after reload.
• A host that starts with a stale cache reads authoritative state inside the coordination boundary and preserves another host's completed write.
• Clear/reset, re-key, prune, profile re-point, and compensation paths do not bypass coordination or overwrite unrelated committed entries.
• Write failures, coordination failures, and retry behavior are explicit and covered; failed operations do not leave phantom cached state or silently remove another host's entries.
• Existing same-host serialization and per-view restoration behavior remain covered.

References

• Parent: Persist per-view view-state identity and durable viewStates in ClineProvider #1536
• PR: test(webview): ClineProvider parallelMode suite with viewStates pruning edges #1921
• Review thread: test(webview): ClineProvider parallelMode suite with viewStates pruning edges #1921 (comment)
• Requested by: @easonLiangWorldedtech

The fork issue easonLiangWorldedtech/Zoo-Code#41 is a working series ledger, not the upstream parent. Upstream #41 is unrelated. This issue tracks a valid, unfixed limitation; filing it does not establish PR merge readiness.