Make incremental iCloud sync truly incremental

[bifrost-proxy]

Summary

• publish clipboard changes as immutable append-only event batches
• use the local incremental state as the only baseline and remove legacy store.json migration/old-client batch-rewrite compatibility
• skip history diffing, batch decoding, state persistence, and UI merging during idle polls
• persist local deltas on quit without waiting for remote checkpoint downloads
• preserve cross-device deletion tombstones while allowing a later genuine recopy

Root cause

The 30-second iCloud poll repeatedly encoded the complete ~52 MB local state, decoded already-applied batches, and merged the full history even when no clipboard data had changed. A second monolithic metadata-cache write on every capture also blocked the main thread long enough to miss rapid clipboard changes.

Validation

• full CI on macOS 26
• idle sync: zero batch decodes and zero persistence
• immutable batch/checkpoint compaction verifier
• three-device deletion, stale-snapshot rejection, and later-recopy regressions
• installed Universal 1.7.2 (44) three-launch regression
• real iCloud three-launch regression via LaunchServices with exact history-count restoration
• File Provider confirms the latest event batch is uploaded and conflict-free

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