Add measured incremental semantic deltas with full-snapshot recovery #547

Parent: #262. Provider prerequisite: #541/#542.

Measure full semantic snapshot cost against unchanged Code OSS and add a bounded incremental delta ABI only where evidence requires it. Deltas must carry base/new snapshot and document/layout generations, deterministic insert/update/remove/reparent/focus/relationship operations, explicit overflow-to-full-snapshot behavior, bounded memory/queues, and navigation/teardown retirement. AppScene consumers must never apply deltas across a generation mismatch.

Acceptance includes large virtualized tree, terminal/editor/panel churn, nested-document replacement, overflow recovery, 100-cycle memory, and p95 publication budgets. This ticket remains after the first native peer consumer establishes real measurements.