[diegosouzapw]

Summary

This PR delivers the independent FU06 Video Bridge slice: it preserves the real chronology of visual observations and supplied transcript cues, keeps each successful caption attached to its source frame, derives the full contact-sheet interval, and rejects an already-aborted fusion before starting either branch.

Implementation base: ee24799e7cb54917fc2a847a90d971402518f359 (release/v3.8.50). The live release branch later advanced by one disjoint commit affecting only config/quality/eslint-suppressions.json; none of the four FU06 paths overlap that drift.

Area	Delivery	Result
Pre-abort	Checks an already-aborted parent signal before starting video or audio callbacks	Neither branch starts after cancellation
Final timeline	Interleaves successful visual captions, transcript, and audioTranscript by time	The rendered description follows media chronology
Transcript array	Sorts the returned transcriptCues chronologically	Structured and rendered timelines agree
Provenance	Uses source: "audio" only for the internal fusion copy	Original client, embedded, and audio-bridge provenance is preserved in the result
Deduplication	Removes exact cues shared by the separately supplied and fusion transcript lanes	A shared cue is rendered and returned once
Partial captions	Keeps successful captions bound to the frame that actually produced them	A failed or empty intermediate caption cannot shift later timestamps
Contact sheet	Uses the minimum and maximum represented frame timestamps	Ordering remains correct even when extracted frame timestamps arrive out of order
Error boundary	Restricts the partial-failure catch to the external caption callback	Internal timeline-construction errors are not silently swallowed

TDD evidence

Regression	RED	GREEN
Already-aborted fusion starts no callbacks	0/1	1/1
Chronological video/audio rendering	0/1	1/1
Failed or empty intermediate caption keeps source timestamp	0/3 semantic assertions	3/3
Full contact-sheet range	0/1	1/1
Combined transcript + audioTranscript timeline	0/1	1/1
Exact cross-lane deduplication	0/1 (2 !== 1)	1/1
Unordered contact-sheet timestamps	0/1	1/1
Chronological transcriptCues array	0/1 (t0.25,t4,t2)	1/1 (t0.25,t2,t4)
Preserve client and embedded provenance	0/1	1/1

Validation

Gate	Result
Focused four-file matrix (videoAudioFusion, provenance, helpers, contact sheet)	PASS — 38/38
Existing describeVideoPart pre-abort regression	PASS — no extraction, broker, or caption start
npm run typecheck:core	PASS
Focused ESLint	PASS
Prettier	PASS
File-size gate	PASS
git diff --check	PASS
Independent Standards/Spec review	PASS — 0 open findings
Broad complexity scan	HOLD / INFRA — local scan reached its execution ceiling with exit 124 and produced no report; this is neither a PASS nor a proven code failure
Full unit/CI merge-candidate matrix	PENDING at PR creation

Scope and independence

• Changes exactly two production files and two regression-test files.
• Does not add STT, remote transcription, cache behavior, log retention, or broker/FFprobe parsing.
• Does not require FU05. FU05 owns embedded-subtitle derivation and retention hardening in a separate branch/PR; FU06 works against the released Video Bridge interfaces and can be reviewed independently.
• Does not include deploy, homologation, merge, tag, or release publication.

PR posture

Keep this PR open in draft while CI runs. This PR does not authorize merge, auto-merge, tag, deploy, or release publication.