perf(spiders): non-blocking async file I/O for cache + checkpoint (#32 #34)

[Liohtml]

Summary

ResponseCache and CheckpointManager did synchronous std::fs I/O while invoked from async contexts — the cache inside per-request tokio::spawn tasks (#32) and the checkpoint in the main crawl loop (#34) — blocking Tokio worker threads on disk I/O.

• get / restore → tokio::fs async reads.
• put / save keep the atomic NamedTempFile::persist write (Windows-correct, from upstream-sync: atomic writes for checkpoint and dev-cache files #31) but run it on the blocking pool via tokio::task::spawn_blocking, since tempfile has no async API. This avoids stalling a worker while preserving atomicity.
• cleanup → tokio::fs::remove_file.
• Engine call sites and the cache/checkpoint unit tests updated to .await (#[tokio::test]).

Test plan

• cache/checkpoint roundtrip + overwrite + cleanup tests pass as #[tokio::test]
• cargo test, clippy --all-targets -D warnings, fmt --check all green

Closes #32
Closes #34

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c4582e0f-555e-408c-855b-700682fd46ac

📥 Commits

Reviewing files that changed from the base of the PR and between 47ac3ce and d07faee.

📒 Files selected for processing (3)

• src/spiders/cache.rs
• src/spiders/checkpoint.rs
• src/spiders/engine.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/async-file-io

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