Streams public API compatibility

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

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

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

The candidate becomes the Streams 1.0 shipped baseline only after the format upgrade suites and the successful 72-hour release-endurance evidence pass. Until then, this is an engineering freeze rather than a claim that 1.0 was published.