checks: add required_topics, the missing-topic QC built-in

[chintondutta]

Fixes #6.

What was missing

src/hflow/checks.py ships built-in checks for timestamps, joints, cameras, idle segments, duration, and digest -- but nothing answered the cheapest and most common corpus-level QC question: did this rig record every topic it was supposed to?

Change

Added required_topics(episode, *, topics), following episode_duration's evidence-only shape (docstring pointing at the curation query where the pass/fail policy lives, no verdict). For each requested topic, records:

• f"{topic}/present" -- boolean
• f"{topic}/message_count" -- int

plus a missing_topic_count summary.

Presence is derived from Episode.channels (dict[int, TopicInfo]), summed across every channel whose topic matches -- never Episode.channel(), which explicitly raises ValueError when a topic maps to more than one channel (episode.py's _resolve_channel_info). A topic counts present when the summed message count is > 0 (an empty channel is schema without a recording, matching episode_duration's own message_count >= 1 presence convention).

No __all__ to update -- checks.py has none; the module is imported and used as hflow.checks.<name>, same as every sibling.

Testing

Two new tests in tests/test_checks.py, using the existing module-scoped jittery_episode synthetic fixture:

• test_required_topics_all_present: /joint_states and /wrist_cam/compressed both present with positive message counts, missing_topic_count == 0.
• test_required_topics_reports_a_missing_topic: a topic name not in the file reports present=False, message_count=0, and missing_topic_count == 1, alongside a genuinely present topic.

uv run pytest -q       # 299 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

[github-actions]

👋 Hi @chintondutta — thanks for the contribution! To keep starter issues available
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