rust cli: Implement mcap get commands

[amacneil]

Summary

• implement mcap get attachment in rust/mcap_cli with --name, optional --offset, and optional --output
• implement mcap get metadata in rust/mcap_cli with --name using namespace-level last-write-wins semantics (highest-offset record for that name)
• wire get subcommand argument structs through clap parsing and command dispatch
• add targeted unit tests for attachment selection and metadata record selection behavior
• fix attachment offset handling so explicit --offset is enforced even when only one same-name attachment exists

Testing

• cd rust && cargo test -p mcap_cli
• cd rust && cargo clippy -p mcap_cli --all-targets -- --no-deps -D warnings

  

[claude]

All prior threads resolved — minimized old review-level comments.

The implementation looks solid. Attachment selection logic is clean, metadata merge semantics match Go, and test coverage hits the important cases. Two minor items inline (neither blocking): the intentional offset-validation divergence from Go, and a missing metadata wiring test case.

One thing I verified: the Rust merged_metadata_for_name explicitly sorts by offset before merging, while Go iterates in MetadataIndexes insertion order. In practice these should match, but the Rust version is more robust if index order ever diverges from file order.

[claude]

Nit: the Rust CLI now validates --offset even when there's only one matching attachment (lines 50-53), which is stricter than the Go CLI (go/cli/mcap/cmd/attachment.go:97-100 — Go silently returns the single match regardless of --offset). I think the Rust behavior is better, but worth noting the divergence. If you want parity, the Go side could get the same fix in a follow-up.

[amacneil]

Rust behavior is better