fix(gps): power saving put the receiver back to sleep before a new fix

MicroNMEA's valid flag survives a power cycle, so the duty cycle saw the
old fix right after waking the receiver and stopped it again a tick
later: with GPS power saving on the position never updated. begin() now
clears the parser.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/helpers/sensors/MicroNMEALocationProvider.h

@@ -86,6 +86,10 @@ public :
 #ifdef GPS_SKYVIEW
         _sky.reset();   // restarts the time-to-first-fix clock
 #endif
+        // MicroNMEA's valid flag is sticky: without this the fix from before the
+        // last stop() still reads valid, and the GPS duty cycle put the receiver
+        // back to sleep a tick after waking it, before any new position came in.
+        nmea.clear();
         claim();
         if (_pin_en != -1) {
             digitalWrite(_pin_en, GPS_EN_ACTIVE);