fix: move HTTP/3 cache I/O off runtime workers

[ryanfowler]

Summary

• move HTTP/3 cache filesystem operations onto Tokio's blocking pool
• return cached candidates before best-effort recency maintenance and rate-limit timestamp writes
• schedule global pruning process-wide in a background blocking task
• update HTTP/3 cache call sites and async tests

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib http3_cache::