chore: add cargo fmt/clippy/test git hooks (.githooks/)

[BryanFRD]

Summary

• .githooks/pre-commit runs cargo fmt --check + cargo clippy -D warnings on commits that touch Rust files.
• .githooks/pre-push runs cargo test --workspace --all-features.
• .githooks/install.sh sets core.hooksPath = .githooks for one-shot setup.
• CONTRIBUTING updated with a one-liner setup instruction.
• .gitattributes enforces LF line endings on the hook scripts so they execute on Windows checkouts too.

Why .githooks/ instead of husky: husky needs a Node toolchain in the repo. FerrFlow is pure Rust + a slim npm wrapper, so the canonical Rust pattern (core.hooksPath) keeps the repo Node-free.

Test plan

• ./.githooks/install.sh set core.hooksPath correctly.
• Pre-commit fired on this commit (no Rust files staged, hook short-circuited as designed).
• Pre-push fired and ran cargo test --workspace --all-features clean (586 passed, 0 failed).

[github-actions]

Benchmark

Details

Benchmark suite	Current: e1da6cd	Previous: 0b5fe57	Ratio
changelog/build_50	8061 ns/iter (± 356)	7955 ns/iter (± 147)	1.01
changelog/build_500	73258 ns/iter (± 3203)	68596 ns/iter (± 1303)	1.07

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