fix(video): validate pass-through fixed GOP cadence

[victorwon2001]

Summary

• Validate pass-through H.264 keyframes against the fixed-GOP message grid derived from configured gop_seconds and each channel's measured frame rate before provenance is written.
• Report off-grid canonical video through the new video-keyframe-cadence doctor error while preserving existing malformed-payload and stream-start diagnostics.
• Add regression coverage for the reported 100-message off-grid stream, exact-grid byte preservation, and independent channels sharing one topic, and document the pass-through requirement and finding code.

Fixes #376

Why

Pass-through episodes previously stamped gop_seconds as the interval actually used without verifying that their H.264 keyframes followed that cadence. Refusing off-grid pass-through streams and checking existing files in hflow doctor makes that provenance assertion verifiable without changing the provenance schema or re-encode behavior.

Validation

Focused and full test suites passed, and lint, formatting, type checking, documentation links, and diff whitespace checks are clean.

uv run pytest -q tests/test_processing_regressions.py tests/test_doctor.py tests/test_transform.py tests/test_video.py
uv run ruff check --fix
uv run ruff format
uv run ty check
lychee --no-progress --include-fragments --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' --exclude-path references/mcap-spec.md --exclude-path references/foxglove-CompressedVideo.proto .
uv run pytest -q
git diff --cached --check

Checklist

• I added or updated outcome-focused tests for changed business logic.
• I updated documentation for changed behavior, flags, formats, or requirements.
• I ran uv run ruff check --fix, uv run ruff format, and uv run ty check.
• I ran the relevant pytest suite.
• I did not add recordings, generated media, credentials, private URLs, or runtime artifacts.
• I preserved stored-data compatibility or documented an explicit version change.