feat: multi-resolution networks (20km/50km) via subpath exports

[mayurrawte]

What does this PR do?

Exposes finer Eurostat maritime networks as subpath exports so consumers can trade download size for coastal-routing accuracy — the design proposed in #11 (unblocked now that #10 ships the network as a shared loadable asset):

import { DEFAULT_MARNET } from 'searoute-ts/marnet-20km'; // or 'searoute-ts/marnet-50km'
import { seaRoute } from 'searoute-ts';

seaRoute(origin, destination, { network: DEFAULT_MARNET });

New subpath exports

Import	Resolution	Segments
searoute-ts/marnet-50km	50 km	15,498
searoute-ts/marnet-20km	20 km	29,581

Each variant re-exports DEFAULT_MARNET (a FeatureCollection<LineString>) plus the MarnetNetwork/MarnetProperties types, and is backed by a single shared data/marnet-<res>.cjs asset that both the CJS and ESM builds load at runtime — the same shared-asset approach #10 introduced for the default network, so a variant is never duplicated across builds and the core stays lean (you only pay for a finer network if you import it).

Generation & provenance

• scripts/build-marnet.cjs now takes an optional resolution label and can generate any Eurostat resolution (5/10/20/50/100 km) from a GeoJSON source; it also (re)emits the matching .d.cts type declaration. scripts/copy-marnet.cjs copies every marnet*.cjs/.d.cts asset into the build outputs.
• The bundled assets were regenerated from the upstream Eurostat GeoPackages (eurostat/searoute, EUPL-1.2). I validated the pipeline by regenerating the 100 km network from source and confirming it reproduces the committed data/marnet.cjs exactly (same 9,847 segments, identical geometry, identical 12 native passage labels), so the 20/50 km variants are produced the same, trusted way.

Not bundled: 10 km / 5 km

The 10 km (~5.9 MB) and 5 km (~9.0 MB) networks are large enough that bundling them would dominate the install, so they are not shipped in the package. They remain generatable with scripts/build-marnet.cjs and loadable via loadNetwork/the network option; the README table documents their sizes and how to use them.

Docs — README gains a size/accuracy tradeoff table (all five resolutions) and an updated bundle-size FAQ; DOCS "Other resolutions" and CHANGELOG updated. No breaking changes; no version bump.

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 ✅ (emits searoute-ts/marnet-20km and marnet-50km in CJS/ESM/types; copies 6 shared network asset files into dist/data)
• npm test ✅ (72 tests; 5 new in src/marnet/index.spec.ts covering the 20 km/50 km feature counts and LineString geometry, ordering vs. the default, preserved native passage labels, routing via the network option, and the finer 20 km path having more coordinates than the default)

Also verified end-to-end against the built dist in both CJS (require) and ESM (import): searoute-ts/marnet-20km loads 29,581 features and Shanghai→Rotterdam routes to ~19,720 km with 244 coordinates versus the default's 112 (same corridor, higher fidelity); marnet-50km loads 15,498 features.

Closes #11

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