Expand benchmarks and clarify range-cache positioning

[xav-db]

Summary

• expand Criterion 0.8 microbenchmarks across cache operations, contention, async read-through, reconstruction, and request coalescing
• add reproducible Apple M4 Pro results plus complete executable sync and async README examples
• raise the MSRV to Rust 1.86, update canonical repository metadata, and add an Ubuntu benchmark smoke job

No cache API or runtime behavior changes.

Validation

• tests with no default features and all features on stable and Rust 1.86
• doctests explicitly with no default features and all features on stable and Rust 1.86
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• rustfmt --edition 2024 --check
• cargo llvm-cov --all-features --workspace --fail-under-lines 95 (96.95% line coverage)
• cargo bench --all-features --bench range_cache -- --test (38 benchmark cases)
• three full benchmark runs on commit 4e9e7baa61ede11bf70b0d246954f3187c1328d8
• cargo publish --dry-run