Sync public API compatibility

The current Sync 1.0 candidate surface is locked by two independent gates:

• Roslyn PublicApiAnalyzers reject undeclared additions and incompatible removals in every Sync package; and
• eng/sync-api-freeze.json records a platform-independent SHA-256 digest for every Sync public API baseline.

This makes an API edit deliberate and reviewable even when it is technically additive. To change the candidate, update the implementation and its API baseline, document source and binary compatibility, run the complete Sync connector and packaging matrices, and update the freeze manifest in the same commit. The compatibility test scans the source tree so a new Sync package cannot omit a frozen baseline.

The candidate becomes the Sync 1.0 shipped baseline only after the 24-hour endurance gate and final connector verification pass. Until then, this is an engineering freeze rather than a claim that 1.0 was published.