Enforce Rust formatting: CI gate, pre-commit hook, rustfmt.toml

[robinskil]

What

Adds tooling so every PR has consistently formatted Rust code, plus a one-time repo-wide reformat so the gate is green from day one.

• CI fmt job (.github/workflows/ci.yml) — runs cargo fmt --all -- --check on every push/PR and fails if any Rust code is unformatted. Lightweight, separate job from clippy/test/build.
• rustfmt.toml — pins the default Rust style (stable-only options). This matches Google's Rust style guide, which mandates rustfmt defaults rather than a custom profile.
• .githooks/pre-commit — local hook mirroring the CI gate; rejects commits with unformatted staged Rust (bypass with --no-verify). Install once with make hooks.
• Makefile — make hooks (sets core.hooksPath) and make fmt targets.
• CONTRIBUTING.md — documents prerequisites (toolchain, submodules, system libs), build/test, and the formatting/clippy/test checks CI enforces; linked from the README.
• Workspace reformat — cargo fmt --all applied across all crates so the new gate passes immediately. Verified formatting-only (identical to a clean rustfmt pass over the base; no logic changes), plus stripping trailing whitespace inside a few #[utoipa::path(...)] macro attributes rustfmt leaves untouched.

Commits

1. Add rustfmt CI gate + pre-commit hook
2. docs: add CONTRIBUTING.md
3. style: format the workspace with rustfmt

Verification

• cargo fmt --all -- --check exits 0 on the branch tip.
• Pre-commit hook verified to both block unformatted staged Rust and pass on clean code.

[codecov]

Codecov Report

❌ Patch coverage is 76.30719% with 290 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (ab1b6ab) to head (f117187).

Files with missing lines	Patch %	Lines
beacon-functions/src/geo/st_within_point.rs	20.00%	36 Missing ⚠️
beacon-mcp/src/catalog.rs	68.91%	23 Missing ⚠️
beacon-core/src/runtime.rs	90.90%	14 Missing ⚠️
...on-functions/src/metadata/view_statistics_cache.rs	0.00%	14 Missing ⚠️
...formats/beacon-arrow-zarr/src/datafusion/source.rs	53.57%	13 Missing ⚠️
...eacon-file-formats/beacon-arrow-tiff/src/reader.rs	36.84%	12 Missing ⚠️
beacon-core/src/statement_plan/physical.rs	64.28%	10 Missing ⚠️
...-functions/src/metadata/view_dataset_statistics.rs	0.00%	10 Missing ⚠️
beacon-core/src/statement_plan/actions.rs	33.33%	8 Missing ⚠️
...acon-arrow-netcdf/src/datafusion/table_function.rs	0.00%	8 Missing ⚠️
... and 49 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #327      +/-   ##
==========================================
+ Coverage   74.51%   74.69%   +0.18%     
==========================================
  Files         290      290              
  Lines       38569    39069     +500     
==========================================
+ Hits        28738    29182     +444     
- Misses       9831     9887      +56     

Files with missing lines	Coverage Δ
beacon-api/src/axum/admin/datasets.rs	88.05% <ø> (ø)
beacon-api/src/axum/admin/mod.rs	87.87% <100.00%> (ø)
beacon-api/src/axum/client/mod.rs	95.12% <100.00%> (ø)
beacon-api/src/axum/client/query.rs	7.35% <ø> (ø)
beacon-api/src/flight_sql/auth.rs	77.35% <100.00%> (-0.83%)	⬇️
beacon-api/src/flight_sql/service.rs	76.97% <100.00%> (+0.46%)	⬆️
beacon-api/src/flight_sql/storage.rs	98.03% <ø> (ø)
beacon-auth/src/basic.rs	97.93% <100.00%> (+0.02%)	⬆️
beacon-auth/src/composite.rs	100.00% <100.00%> (ø)
beacon-auth/src/role.rs	97.79% <100.00%> (+<0.01%)	⬆️
... and 124 more

... and 1 file with indirect coverage changes

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