Fix false storm alarm from unwrapped own-ship position node (fixes #3)

[fakehec]

Fixes #3.

Problem

vessel() reads own-ship position with app.getSelfPath('navigation.position') and passes it through as { position, sog, cog }. On Signal K servers where getSelfPath returns the full node ({ value: { latitude, longitude }, timestamp, … }) rather than the bare value, vessel.position.longitude/.latitude are undefined. StormEngine.evaluate then builds vp = [undefined, undefined], distance-to-cell is null, and pathPolygonThreat degenerates to a spurious intersection — promoting distant cells to a false alarm with a 0.0 NM away, CPA NaN, intersects in 0 min message.

Self-contained reproduction is in #3.

Fix

• vessel() now unwraps .value for position, sog and cog, so both the node and bare-value shapes work.
• StormEngine.evaluate derives the vessel point via a small vesselPoint() helper that also unwraps .value and requires finite longitude/latitude. A missing or malformed position therefore yields vp = null (distance null ⇒ severityState returns normal) instead of a fabricated zero-distance intersection. This keeps the engine safe regardless of caller shape.

Tests

New test/storm-engine-vessel.test.js:

• far cell is normal with a plain {latitude,longitude} position;
• node-form {value:{…}} position is unwrapped and yields the same result (no false alarm, identical finite distance);
• positions without finite coordinates degrade to normal with no fabricated distance;
• a genuinely close severe cell still raises the alarm (from a node-form position).

npm test and npm run lint are clean.

Impact

Affects any storm source (including the bundled Radar-DPC HRD) on servers whose getSelfPath returns nodes. No behavior change on servers that already return bare values.