feat(gps): add fix-quality (accuracy) tier to GPS sensor fusion

Extend LocationService's source-selection policy with a quality-aware
"marginal staleness" zone between the primary and a new extended
staleness threshold (default 10 s):

  1. Fresh NMEA (≤ primary threshold, 5 s)  →  always prefer NMEA
  2. Marginally stale NMEA (5–10 s)         →  prefer NMEA only when
     GpsPosition.accuracyMeters is strictly better than Android's;
     fall back to Android conservatively when accuracy data is absent
  3. Very stale NMEA (> 10 s)               →  always prefer Android
  4. Only one source available               →  use it regardless of age

Changes:
- GpsPosition: add nullable accuracyMeters field (default null, no
  breaking change to existing callers)
- LocationService: add nmeaExtendedThresholdMs constructor parameter;
  recomputeBestPosition() now implements three-tier logic; extract
  GpsPosition.hasStrictlyBetterAccuracyThan() helper
- LocationServiceTest: expose nmeaExtendedThresholdMs in fusionService
  helper; add posWithAccuracy helper; add 4 new test cases covering
  accuracy-based NMEA preference, worse-accuracy fallback, no-accuracy
  conservative fallback, and very-stale unconditional fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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