docs: document doctor finding codes; correct the B-frame conformance claim

[chintondutta]

Fixes #10.

Two gaps

1. docs/FORMAT.md claimed a check the doctor doesn't perform. Conformance item 4 stated "no B-frames" as part of what hflow doctor checks. src/hflow/doctor.py verifies the h264 format, AUD-delimited access units, SPS/PPS on keyframes, and keyframe-first ordering -- but has no B-frame detection. Reworded the item to keep no-B-frames as a stated convention (linked to the existing "In-band video" section that already documents it) while being explicit that the doctor doesn't check it yet.

2. The doctor's finding codes were documented nowhere. Finding.code is declared a "stable kebab-case identifier" but no page listed them. Enumerated all 16 by grepping collector.add( in doctor.py (the issue estimated ~14; there are 16) and added a table under a new "Doctor finding codes" section in docs/FORMAT.md: code, level, one-line meaning, matching each call site's actual message.

Docs only, no code changes.

Testing

docker run --rm -v "$PWD:/data" -w /data lycheeverse/lychee:latest --no-progress --include-fragments \
  --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' \
  --exclude-path references/mcap-spec.md \
  --exclude-path references/foxglove-CompressedVideo.proto .
# 231 Total, 0 Errors

uv run pytest -q       # 297 passed, 3 skipped; unrelated: tests/test_ffmpeg.py fails/errors in this sandbox (no ffmpeg/ffprobe on PATH)
uv run ruff check --fix
uv run ruff format
uv run ty check

(lychee wasn't on PATH in this sandbox; ran it via the official Docker image instead of installing.)

[github-actions]

👋 Hi @chintondutta — thanks for the contribution! To keep starter issues available
for other contributors and give every pull request a real review, we accept
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You already have #52 open, so this one is being closed automatically.
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