feat(geo): add geographic foundations

[404khai]

Summary

Implements Phase 3 of the Roadrunner roadmap with validated geographic values and straight-line distance calculations in roadrunner-core.

• adds validated WGS 84 Coordinate values
• adds inclusive axis-aligned BoundingBox values
• adds Meters, Seconds, and KilometersPerHour domain types
• exposes Distance as Roadrunner's canonical meter-based distance type
• implements haversine_distance(Coordinate, Coordinate) -> Meters
• extends graph Node values with validated coordinates
• updates Phase 2 graph fixtures and traversal benchmarks for the new node contract

Validation boundaries

Coordinates reject non-finite values, latitudes outside [-90, 90], and longitudes outside [-180, 180].

Physical units reject negative, NaN, and infinite values. Their Serde implementations deserialize through the same validated constructors instead of bypassing invariants.

Bounding boxes require a south-west corner that does not exceed the north-east corner on either axis. Antimeridian-crossing boxes are explicitly deferred and rejected by this initial axis-aligned representation. Bounding-box deserialization also revalidates these invariants.

Haversine implementation

The calculation uses the Haversine formula with the mean Earth radius and clamps the intermediate Haversine value to [0, 1] to contain floating-point drift before the inverse trigonometric step.

Tests cover:

• zero distance for identical coordinates
• an equatorial quarter circumference
• the known London-to-Paris distance
• distance symmetry

The function returns Meters directly and represents straight-line surface distance, not road-network travel distance.

Graph integration

Node::new now requires a validated Coordinate, and nodes expose that coordinate through a typed accessor. Existing topology tests and the generated graph benchmark use Coordinate::ORIGIN, keeping benchmark behavior focused on adjacency traversal.

No edge cost, travel-time calculation, or routing algorithm is introduced; those remain assigned to later phases.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked — 27 tests pass
• cargo bench -p roadrunner-core --bench graph_traversal --no-run --locked
• RUSTDOCFLAGS=-Dwarnings cargo doc --workspace --no-deps --locked
• git diff --check main...HEAD