feat: add antimeridian output option for dateline-crossing routes

[mayurrawte]

Summary

Routes crossing the ±180° antimeridian (e.g. Yokohama → LA) come back wrapped to [-180, 180], so a segment jumps from +179 to -179. Fed straight into MapLibre / Leaflet / Deck.gl, that draws as a straight streak across the whole map instead of across the Pacific. This was already solved for our own demo (examples/web-demo/src/geo.ts); this PR moves the capability into the library so downstream map consumers don't each rediscover it.

What changed

New antimeridian option on seaRoute and seaRouteMulti:

seaRoute(yokohama, la, { antimeridian: 'unwrap' }); // continuous coords (may exceed ±180)
seaRoute(yokohama, la, { antimeridian: 'split' });  // MultiLineString split at ±180

• Default undefined — current behaviour, wrapped LineString (no breaking change).
• 'unwrap' — one continuous LineString; longitudes shifted by multiples of 360° so consecutive points never jump the dateline (the demo's approach).
• 'split' — a MultiLineString cut at ±180°, keeping every coordinate within ±180° (RFC 7946-friendly), inserting the interpolated dateline point at both ends of each cut.

Details:

• New src/lib/antimeridian.ts with unwrapCoords / splitAtAntimeridian.
• Return types are narrowed by overload: { antimeridian: 'split' } returns the new SeaRouteMultiFeature (Feature<MultiLineString, …>); everything else still returns SeaRouteFeature. So existing TS consumers see no type change.
• seaRouteMulti applies the option once to the concatenated route (legs are computed as plain wrapped LineStrings so they still join cleanly).
• seaRouteAlternatives continues to return wrapped LineStrings (documented).
• properties.length and bbox are computed from the geodesic path and are unaffected by the representation.
• New exports: Antimeridian, SeaRouteMultiFeature. README / DOCS / CHANGELOG updated.

One subtlety worth noting

The bundled network normalizes lon === 180 vertices to -180, so a trans-Pacific route can pass through a vertex sitting exactly on the dateline. splitAtAntimeridian works in unwrapped space and assigns each sub-line a 360°-panel offset, so boundary-coincident vertices are handled as crossings (and de-duplicated) rather than being dropped — verified against Yokohama → LA (vertex on 180) and Sydney → Vancouver (interpolated crossing at lat 0).

Validation

All steps CI runs, from a clean tree:

• npm ci
• npm run lint ✅
• npm run format:check ✅
• npm run build ✅
• npm test ✅ (50 tests; 6 new covering unwrap continuity, split cut points at ±180 with continuous latitude, all coords within ±180, non-crossing routes unchanged, seaRouteMulti split, and alternatives staying LineString)

Also verified end-to-end against the built dist (CJS + ESM) that unwrap removes all dateline jumps and split produces valid in-range MultiLineString geometry with length preserved.

Closes #9