Make acknowledged MCP frame navigation atomic

[CameronFoxly]

Summary

• add a hook-owned, awaitable frame navigation operation that flushes the installed canvas into its old content frame before moving the playhead
• install populated or empty target-frame data into canvasStore and advance synchronization refs before returning applied.currentFrameIndex
• reuse the serialized browser command dispatcher from Acknowledge serialized MCP browser commands #156 so immediately following current-frame batches cannot outrun navigation
• preserve explicit set_cells_batch.frameIndex active/background targeting

Regression coverage

• flush / playhead / load / acknowledgement ordering
• populated and empty targets
• invalid indices and failure propagation
• pending layer switches
• variable-duration go_to_frame(5) immediately followed by a targetless mutation, with both acknowledgements reporting timeline frame 5 while content ordinal 1 is mutated

Validation

• focused MCP/navigation tests (26 tests)
• adjacent synchronization suite (211 tests)
• targeted ESLint
• production build

Stacked on #156. Addresses #154.