feat(gps): use HDOP for !gps fix readiness, satellite count as fallback

Satellite count alone is a poor proxy for fix quality -- few satellites
in good geometry can beat many in poor geometry. LocationProvider now
exposes getHDOP() (default -1 = unsupported); MicroNMEA implements it.
isLocFixReady() prefers HDOP <= 2.0 when available, falling back to the
old >=8 satellite threshold for providers that don't report it (e.g.
RAK12500/u-blox).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/helpers/sensors/LocationProvider.h

@@ -14,6 +14,12 @@ public:
     virtual long getLongitude() = 0;
     virtual long getAltitude() = 0;
     virtual long satellitesCount() = 0;
+    // Horizontal Dilution of Precision, in tenths (11 == HDOP 1.1) -- lower is
+    // better, and a much more direct read on fix quality than satellite count
+    // alone (few satellites in good geometry can beat many in poor geometry).
+    // -1 means this provider doesn't expose it; callers fall back to
+    // satellitesCount() in that case.
+    virtual long getHDOP() { return -1; }
     virtual bool isValid() = 0;
     virtual long getTimestamp() = 0;
     virtual void sendSentence(const char * sentence);

src/helpers/sensors/MicroNMEALocationProvider.h

@@ -120,6 +120,7 @@ public :
         return alt;
     }
     long satellitesCount() override { return nmea.getNumSatellites(); }
+    long getHDOP() override { return nmea.getHDOP(); }
     bool isValid() override { return nmea.isValid(); }
 
     long getTimestamp() override {