Accept Flow usage and cache key at Factory ingress

[flujo-app]

FLUJO's real chat SDK request includes stream_options.include_usage=true and prompt_cache_key. The strict receiver previously returned 422 before Factory admission. This PR accepts the explicit usage option on chat and legacy completions, emits a terminal usage chunk only after a successful stream, and accepts a bounded opaque cache key on chat. The key is included in Factory's normalized-body admission digest. Ordinary mixed-version text peers receive their existing request shape; this receiver does not implement prompt caching.

The saved real Flow R2 SDK body is pinned at 299 UTF-8 bytes and SHA-256 b86e125b6a031a226acc03147b5d865402ad149a621626fcdbf7629f5541fd03. An offline ASGI fixture checks its independently fixed 307-byte normalized body and reaches a synthetic Factory claim that holds before physical dispatch. Changing the key fails body admission; malformed keys fail schema validation. This qualifies receiver grammar and the held path only. Authenticated raw-wire/identity binding, a versioned receiver profile, durable production Original claims, coherent worker generation, and physical sends remain separate work.

This draft is stacked on #36, the CI baseline repair. Local validation: integrated Factory focused checks 134 passed/1 skipped, broad offline selection 1,028 passed/13 skipped, and cache-key candidate focused checks 80 passed; pinned Black/isort and diff checks passed. Hosted validation on exact head 5dec495: Black, isort, Ubuntu/macOS broad tests, and Linux/Windows packaging protocol tests and desktop self-tests pass. Both packaging jobs then fail at the unsigned public-alpha bundle because the signed sequence-2 Qwen catalog expired on 2026-09-28. #37 is an unsigned review-only sequence-3 renewal candidate, not a publication.