cli: add Apache Arrow IPC support to mcap convert

[amacneil]

Summary

Adds Apache Arrow support to the Rust CLI for the arrow message/schema encodings proposed in #1748, on both the write and read paths:

• mcap convert reads an Arrow IPC file or stream (.arrow, .feather, .ipc, .arrows) and writes a single channel using the arrow message encoding, with the Arrow IPC Schema stored once in the Schema record (arrow schema encoding). Each input row becomes one message containing a bare encapsulated RecordBatch (matching pyarrow.RecordBatch.serialize()) — no stream framing, no embedded schema, no Arrow body compression. Dictionary-encoded columns are hydrated to their value type so every RecordBatch decodes independently.
• mcap cat --json now decodes arrow messages: it reads each RecordBatch against the channel's Schema record and emits the rows. Because a message may hold multiple rows, the data field is a JSON array of row objects.

Timestamp handling

log_time/publish_time are derived from Arrow columns:

• --log-time-field <NAME> selects the column; otherwise a column named log_time is used, falling back to the first timestamp/date column in schema order (Arrow field order is well-defined and preserved through IPC).
• --publish-time-field <NAME>, then a column named publish_time, otherwise defaults to log_time.
• Arrow Timestamp/Date32/Date64 columns are scaled from their intrinsic unit to nanoseconds. Integer columns use --timestamp-unit {s,ms,us,ns} (default ns).
• Null, negative, or out-of-u64 times are hard errors.

New flags on convert

--topic (defaults to the input file stem), --schema-name, --log-time-field, --publish-time-field, --timestamp-unit, --rows-per-message (default 1, for opt-in row batching).

Dependency

Adds arrow (ipc, json, chrono-tz features). json powers cat --json via arrow-json's writer; chrono-tz is required so named timezones (e.g. timestamp[us, tz=UTC]) render correctly.

Testing

• 14 new unit tests (13 for convert in arrow_ipc.rs, 1 for cat --json in cat.rs). The convert tests generate Arrow IPC fixtures dynamically (no committed binaries / LFS), covering unit scaling, auto-detection, named/explicit/default field resolution, dictionary hydration, row batching, stream vs file framing, multi-batch files, and error paths; each round-trips the emitted bytes back through an Arrow StreamReader.
• Manual end-to-end: converted a timestamp[us, tz=UTC] + dictionary Arrow file and a real third-party Arrow file (samplefile.com), verified with mcap info/list, mcap cat --json, and a pyarrow round-trip decode.

Depends conceptually on #1748 (registry entry); this PR is the implementation.

  

[claude]

Solid converter, tests round-trip through a real StreamReader which is the right way to prove spec compliance. A few things to settle, mostly inline.

One risk that doesn't pin to a single line: Arrow's time handling fails hard mid-stream (null, negative, or out-of-u64 values are errors). Unlike the bag/db3 paths, a bad value deep in a large file aborts after the output .mcap has already been partially written and never finalized, leaving a truncated file at the output path. Worth confirming that's acceptable, or cleaning up the output on error.

Open question on sequencing: this rides on #1748 for the arrow encoding registration — see the docs comment. Draft is the right state until that lands.

[claude]

This registry#arrow anchor — and the arrow message/schema encoding names this converter emits — depend on #1748, which isn't merged. Until the registry entry is published, this link 404s and we'd be shipping files with an unregistered encoding. Make sure this doesn't land ahead of #1748.

[claude]

New commit closes last round's decoder-reuse coverage gap — arrow_json_reuses_cached_decoder_across_messages runs two messages through one JsonTranscoders and asserts the second decodes via the cached decoder. 👍

Still gated on #1748: files written with the arrow encoding (and the registry#arrow doc link) can't land until that registry entry is published. Open thread on cli.md tracks it — draft is the right state until then.