Add GetTransaction/CancelTransaction to npackd

[imattau]

Summary

• Install and Update accept "async": true to return {"transaction_id": N} immediately instead of blocking until completion.
• New GetTransaction { transaction_id } polls a transaction's status: running, succeeded (with result), failed (with error), or cancelled.
• New CancelTransaction { transaction_id } requests cancellation — cooperative, not forcible. A shared AtomicBool cancellation flag is threaded through InstallRefOptions/ResolverState and checked only at the start of processing each package (before starting the next package in a dependency graph, or the next package in an Update loop) — never mid-download or mid-install of a package already in progress, so a cancelled transaction cannot leave the store half-installed.
• Synchronous (non-async) Install/Update calls are unaffected — same behavior as before, just now with an unused cancel: None.

Why cooperative cancellation

The previous PR's follow-up notes flagged that raw tokio task abortion is unsafe here: aborting mid-file-write or mid-manifest-write could leave the package store in a half-installed, inconsistent state. This checks a flag only at package boundaries, which is the same "unit of work" granularity the existing dependency-cycle/already-installed checks use.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features (no warnings)
• cargo test (50 passing), including:
  • install_remote_package_respects_cancellation_before_starting_a_package — a pre-cancelled flag causes an immediate, clean bail before any network/filesystem work
  • transactions_report_running_then_succeeded_and_reject_unknown_ids
  • cancel_transaction_stops_a_cooperative_task
• Manual smoke test over a real socket:
  • Async Update against an empty store → {"transaction_id":1} → GetTransaction reports succeeded with the same array Update would have returned synchronously
  • Async Install of npack's own real release against a real relay, cancelled immediately → GetTransaction reports cancelled before any download started

Not in this PR

Streamed progress events (e.g. per-file download progress) — a client still only sees running until the transaction finishes, not incremental progress. Tracked in the roadmap as remaining Phase 2 work.

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