fix(video): make one-frame scene sampling deterministic

[diegosouzapw]

Summary

• Make scene_aware Video Bridge sampling deterministic when the caller allows only one frame.
• Fall back to the midpoint of the active full-video or focus window and report policyEffective: "uniform", because one scene candidate cannot preserve both temporal ends.
• Add an opt-in real-FFmpeg fixture matrix covering rapid edge cuts, a one-frame budget, static and gradual scenes, sub-second clips, and detector failure.

Related Issues

• Related to feat(backend): Video bridge (frame sampling + captioning) - backlog da série Modality Bridge #9760

Validation

Choose the change type and focused loop from the
Contribution Golden Path. The full unit suite,
Vitest, the 60% coverage gate, and the production build all run in CI on this PR (#8329):

• Change type: other — Video Bridge guardrail/runtime
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Post-rebase evidence against the then-live release/v3.8.50 tip:

• Video Bridge unit matrix: 42/42 PASS.
• Real FFmpeg integration matrix: 7/7 PASS with physical FFV1/Matroska fixtures.
• npm run typecheck:core: PASS.
• npm run typecheck:noimplicit:core: BASE-RED outside this PR in
  open-sse/translator/response/openai-responses/pureHelpers.ts (implicit-any
  diagnostics from the current release tip); this branch changes neither that file nor
  the no-implicit config.
• npm run check:docs-all: PASS; only the existing soft executor-count/version notices were reported.
• Focused ESLint, Prettier, changelog integrity, and git diff --check: PASS.
• The full npm run lint passed before the final base-only rebase; the affected files were linted again after rebase.

Tests Added Or Updated

• tests/unit/guardrails/videoBridgeSampler.test.ts
• tests/integration/video-bridge-sampler-ffmpeg.test.ts

The real-FFmpeg matrix is intentionally opt-in because FFmpeg is an optional operational dependency:

RUN_VIDEO_BRIDGE_FFMPEG=1 node --import tsx/esm --test \
  tests/integration/video-bridge-sampler-ffmpeg.test.ts

Without the environment variable, the integration collector records an explicit skip instead of hiding the missing runtime dependency.

Coverage Notes

• The unit regression proves the exact former failure: candidates at 0.25s and 7.75s with a one-frame budget previously selected the final candidate and still claimed scene_aware; it now selects the deterministic 4s midpoint and reports uniform.
• A focused-window case proves [2s, 8s] resolves to 5s and retains the normalized focus metadata.
• The integration test executes real ffprobe/ffmpeg commands and verifies extraction, not only selection helpers.
• This slice does not change segment_aware behavior or the FU-07 structural sampler.

Reviewer Notes

• Scope is limited to the FU-02 one-frame acceptance gap. It does not touch Video Bridge cache identity, subtitle extraction, Audio Bridge fusion, drill-down, contact-sheet composition, or FU-07.
• No migration or feature flag is involved.
• The active release branch is moving quickly; the branch was rebased and the focused gates above were rerun immediately before publication.

[diegosouzapw]

Exact-base CI classification (snapshot: 2026-08-24 07:17 UTC; base dafb4ae808305c9287c751cf1ce16fe2234ba073, head 6f8a2a348b7be6c3edb6c0903e4b9f8f9c0d8e5e):

• Unit Tests fast-path (1/4) — BASE-RED: the PR reports OpenAPI coverage 34.5% (344/996) versus the 34.6% floor. Running the same test on the exact base produced the same 34.5% (344/996) result; this PR does not change the OpenAPI/routes/test surface.
• Fast Production Build — INFRA/cancelled: the runner was shut down during the optimized production build, without a compile/type diagnostic. The exact-base build was cancelled at the same phase.
• DAST — not started: the runner terminated the prerequisite CLI build; server startup and the Schemathesis/promptfoo checks were never reached.
• The release branch remains independently base-red in 🔴 Release branch not green: release/v3.8.50 #9985 (releaseGreen:false, unrelated ESLint JSON parsing failure).

No PR-specific defect is proven by these failures, so no noise commit was added. This is not a claim that the full CI suite is green; the PR remains open and draft.