episode: expose ChannelData.publish_times

[chintondutta]

Fixes #4.

What was missing

ChannelData stores per-message publish times but never exposed them: the constructor accepts publish_times and assigns self._publish_times (src/hflow/episode.py), and Episode.channel() faithfully concatenates them from the reader -- but there was no property, so the data was plumbed end to end and then unreachable.

Change

Added a publish_times property directly below timestamps, following the same pattern, returning self._publish_times. The docstring notes these are publish times (not log times) and, unlike timestamps, are not guaranteed ascending.

Testing

New test test_channel_publish_times_are_reachable (tests/test_end_to_end.py), using the existing synthetic-episode fixture: asserts ep.channel("/joint_states").publish_times is an np.ndarray with shape (len(channel),).

uv run pytest -q       # 298 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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You already have #52 open, so this one is being closed automatically.
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