Add caching for crates.io metadata

[timweri]

Cache crates.io metadata so we don't have to fetch it every time.

[Copilot]

Pull request overview

Adds a persistent JSON-backed cache for crates.io API responses so that publish dates and per-crate version listings can be reused between runs, and wires the cache through the CLI and the GitHub Action.

Changes:

• New src/cache.rs module providing ResponseCache with TTL-bounded all-versions caching, indefinite publish-date caching, atomic save via tmp+rename, and tests.
• CratesIoClient is now stateful (&mut self), consults/populates the cache, and exposes rate_limit() (skipping the 100 ms sleep on cache hits) and save_cache().
• CLI gains --cache-path (with CARGO_OXIDATE_CACHE_PATH env) and --cache-max-age-hours; the composite action adds a cache-responses input that provisions actions/cache keyed on Cargo.lock.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/cache.rs	New persistent cache module with serialization, TTL, atomic save, and tests.
src/api.rs	Threads cache + TTL + last_was_cache_hit through fetch methods; splits uncached helpers; adds save_cache/rate_limit.
src/main.rs	Wires CLI cache options into CratesIoClient; replaces inline sleeps with client.rate_limit(); calls save_cache() at end.
Cargo.toml	Adds clap env feature, serde_json runtime dep, tempfile dev-dep.
Cargo.lock	Lockfile updates for the new deps and their transitives.
action.yml	Adds cache-responses input, resolves cache path, uses actions/cache, passes CARGO_OXIDATE_CACHE_PATH.
README.md	Documents new CLI flags, caching section, and cache-responses action input.

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