perf: load the network from a single shared asset instead of inlining it in both builds

[mayurrawte]

Summary

The Eurostat marnet was inlined as a ~1.1 MB JSON string in src/lib/marnet.ts and compiled into both the CJS and ESM builds, so the published tarball shipped the network twice (~2.2 MB of identical data). This was already flagged as a follow-up in CHANGELOG.md.

This PR extracts the network to a single shared asset that both builds load at runtime, rather than a source module compiled into each.

What changed

• data/marnet.cjs — the network now lives in one committed CommonJS data module (module.exports = JSON.parse(...)), with a companion data/marnet.d.cts type declaration.
• src/lib/marnet.ts — slimmed to import marnetData from '../../data/marnet.cjs' and re-export. The specifier resolves to a single dist/data/marnet.cjs at runtime from both dist/cjs/lib and dist/esm/lib (../../data/marnet.cjs → dist/data/marnet.cjs in both).
• scripts/build-marnet.cjs — updated to generate data/marnet.cjs instead of src/lib/marnet.ts.
• scripts/copy-marnet.cjs (new) — copies the asset into dist/data (build) and build/data (tests).
• package.json — build and build:test copy the asset into place; files already ships dist.
• Docs: CHANGELOG entry, README bundle-size FAQ, and the old follow-up note marked done.

Why a .cjs data module (not fs or JSON import)

• A .cjs file is always CommonJS regardless of the surrounding package.json type, so CJS require and ESM import of it both resolve the single asset across Node 18/20/22 — no import.meta vs __dirname split, no version-fragile JSON import attributes (assert vs with), and no fs (so it stays bundler-friendly).

Impact

• ~50% smaller install: ~329 KB → ~184 KB packed, ~2.5 MB → ~1.3 MB unpacked (npm pack --dry-run).
• Data ships once (dist/data/marnet.cjs); the per-build marnet.js files are now a few hundred bytes each.
• No public API change. Prerequisite for Optional multi-resolution networks (5/10/20/50 km) via subpath exports #11 (multi-resolution networks via subpath exports).

Validation

All steps CI runs, from a clean tree:

• npm ci
• npm run lint ✅
• npm run format:check ✅
• npm run build ✅
• npm test ✅ (45 tests; added DEFAULT_MARNET loads the shared network asset)

Additionally verified both published entrypoints load the single asset and route identically:

• require('./dist/cjs/index.js') → 9847 features, Shanghai→Rotterdam 10666 nm
• import('./dist/esm/index.js') → 9847 features, Shanghai→Rotterdam 10666 nm

Closes #10