feat: add via option to force routes through named passages

[mayurrawte]

What does this PR do?

Adds a via option to seaRoute — the inverse of restrictions. restrictions blocks a passage; via requires one, so users can force and compare explicit routings (e.g. "via Suez" vs "via Panama"):

seaRoute('CNSHA', 'NLRTM', { via: ['suez'] });   // through Suez (the default) — 19,753 km
seaRoute('CNSHA', 'NLRTM', { via: ['panama'] });  // across the Pacific + Panama — 25,315 km

Design (as suggested in the issue — reuse the multi-leg machinery):

• Routes origin → passage₁ → … → passageₙ → destination, snapping each passage's PASSAGE_BBOXES centroid to the network as an intermediate waypoint via seaRouteMulti. No new routing logic. New passageCentroid() helper in restrictions.ts.
• via accepts the same Passage names as restrictions and visits multiple passages in the order given.
• A passage named in via is dropped from the effective per-leg restriction set, so a required passage is never blocked out from under the requirement — via: ['northeast'] reaches the Northeast Passage without also needing allowArctic.
• Naming a passage in both via and restrictions (compared after alias canonicalisation, so via:['babalmandab'] + restrictions:['babelmandeb'] counts) is a contradiction and throws NoRouteError.
• greatCircleLength/detourRatio stay measured against the direct origin→destination geodesic, matching plain seaRoute.

Acceptance criteria (from #8)

• ✅ via accepts the same Passage names as restrictions.
• ✅ Combining via and restrictions with a contradiction throws NoRouteError.
• ✅ Test: Shanghai → Rotterdam via: ['panama'] routes across the Pacific + Panama, not Suez.

Checklist

• npm run lint and npm test pass
• New behavior is covered by a test
• CHANGELOG.md updated (user-visible changes only)

Validation (all steps CI runs, from a clean tree)

• npm ci
• npm run lint ✅
• npm run format:check ✅
• npm run build ✅
• npm test ✅ (74 tests; 7 new covering via-Panama across the Pacific/not-Suez, via-Magellan rounding Cape Horn, via-Suez staying on the default route, via-Northeast without allowArctic, the origin→destination great-circle reference, and both contradiction cases including the alias)

Also verified end-to-end against the built dist (CJS): via:['panama'] = 25,315 km through Panama with no Suez, via:['suez'] = 19,753 km, and the contradiction case throws NoRouteError.

Closes #8

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