docs: docstrings for CanonicalMcapWriter public methods and DiagnosticLevel

[chintondutta]

Fixes #11.

The gap

CanonicalMcapWriter is exported in hflow.__all__, but its public methods had no docstrings -- the only exported class in the API with that gap: register_schema, register_channel, write_message, add_metadata, add_attachment, finish. DiagnosticLevel was the only exported class with no class docstring (its members were commented inline instead).

Change

Added one docstring per method stating its contract (what it returns, what must already be registered, ordering rules), matching the existing tone in the codebase (e.g. ChannelData properties in episode.py, CanonicalMcapWriter.abort() in the same file): one line of contract plus notable caveats, not a restatement of the implementation. finish()'s docstring in particular documents the ordering rule -- must be called last, every other public method raises afterward -- since the guard (_raise_if_finished) is the enforcement but wasn't stated as a contract anywhere. Added a one-line class docstring for DiagnosticLevel.

No behavior changes.

Testing

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

[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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