Correct the compass with the error measured against the GPS course

[rodrigohpalmeirim]

The compass takes over the heading when we stop, but it's often off by tens of degrees and sometimes reversed outright (uncalibrated magnetometer, steel of the bike and mount). While riding we have a trusted heading from the GPS course, so this measures the compass error along the current leg and takes it out when the compass has the say.

Offset estimator (src/lib/compass-offset.ts)

• Every raw compass reading (before the 2° / 200 ms throttle) and every fix course at ≥2 m/s is fed to the estimator; the course is only paired with the compass if it held steady (within 8°) over the previous 2 s, since the GPS course lags the compass by about a fix and pairing them mid-turn is meaningless
• The offset is a circular exponentially weighted mean (~10 s memory) of course − compass, using the compass average over the stability window to shed jitter
• Applied only once ≥3 samples agree closely (resultant length ≥ 0.95, roughly 18° of scatter); a phone held at a random angle to the bike never gets past that gate, and a reversed compass is corrected like any other offset
• Reset on any visibilitychange: the screen is kept awake during a trip, so it going dark means the phone was pocketed, where it points anywhere but forward. What was learned before doesn't hold in there, and what's learned in there doesn't hold once it's out

Wiring (location.ts, compass.ts)

• Only the fix's own heading is used as a calibration reference, never the anchor-derived bearing between fixes
• On compass takeover currentHeading receives the corrected value, which also lands close to the last GPS course so the marker no longer snaps at the handover

Test plan

• npm run check, eslint on touched files, vitest run (9 new tests for the estimator; full suite passes)
• On-bike testing: correction should kick in a few seconds after stopping, marker shouldn't snap at takeover
• Pocket the phone mid-trip, take it out at a stop: heading should be uncorrected raw compass until riding again