fix: coalesce comm_msg state updates within a flush cycle

[cpsievert]

Summary

Fixes #212 — ipyleaflet LayersControl losing track of layers when reactive effects do remove+add within a single flush cycle.

Root cause: When multiple reactive effects modify the same widget (e.g., map.remove(old_layer) then map.add(new_layer)), each trait change sends a separate comm_msg to the client. The intermediate states (e.g., "layers after remove but before add") cause race conditions in ipyleaflet's async JS layer management. The final map rendering was correct, but the LayersControl overlay list would lose entries.

Fix: State-update comm_msg messages (method="update") for the same model are now merged within each flush cycle, sending only the final value of each changed trait. This is equivalent to wrapping every reactive effect in ipywidgets' hold_sync(), but applied automatically at the transport layer. Non-update messages (method="custom") are still sent individually.

Key changes:

• _comm.py: Added coalescing logic in _publish_msg — first update for a comm_id registers an on_flushed callback; subsequent updates merge their state dict into the pending entry
• Buffer paths for overridden state keys are properly replaced during merges
• comm_close and non-update comm_msg messages are unaffected

Test plan

• 5 new unit tests covering: basic coalescing, cross-trait merging, buffer handling, non-update passthrough, independent coalescing per model
• New Playwright test (ipyleaflet_layer_update) that verifies LayersControl stays consistent after remove+add cycles
• All 90 unit tests pass
• All existing Playwright tests pass (marker click, geojson hover, rerender cleanup)
• make check passes (format + types + unit tests)

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