Changes:
- Add Rust caching (Swatinem/rust-cache) to all heavy build jobs
- Reduce cargo-check-each-crate from 7 to 4 parallel jobs
- Reduce tests-linux-stable matrix from 6 to 3 jobs
- Set CARGO_INCREMENTAL=0 for consistent caching
- Reduce timeouts from 60 to 45 minutes (cache makes builds faster)
- Remove redundant disk cleanup steps (cache handles this)
Expected improvements:
- 50-80% faster builds after cache is populated
- Lower VPS load (fewer parallel jobs)
- More consistent build times
Affected workflows:
- checks.yml (cargo-clippy, check-try-runtime)
- tests.yml (quick-benchmarks, cargo-check-all-benches)
- tests-misc.yml (test-pezframe-ui, cargo-check-each-crate)
- tests-linux-stable.yml (test-linux-stable)
- docs.yml (test-doc, build-rustdoc)
