fix: GPS shutdown active-level, buzzer octave-8 consistency, waypoint header read checks

- UITask::shutdown(): power GPS off using !PIN_GPS_EN_ACTIVE instead of hardcoded
  LOW, matching MicroNMEALocationProvider::stop() (still LOW on current active-high
  devices, but correct for a future active-low GPS).
- buzzer _noteFreq(): clamp octave to 8 instead of 7 so the parser's accepted range
  (4-8) is fully consumed; previously an octave-8 digit was clamped away and could
  leak into the stream as the next note's duration.
- Waypoint::readFrom(): check read() return for version/reserved/count header bytes
  so a truncated file is rejected instead of using garbage count.

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

src/helpers/ui/buzzer.cpp

@@ -41,7 +41,7 @@ uint16_t genericBuzzer::_noteFreq(char letter, bool sharp, uint8_t octave) {
     uint8_t idx = NOTE_IDX[letter - 'a'];
     if (sharp) { if (++idx >= 12) { idx = 0; octave++; } }
     if (octave < 4) octave = 4;
-    if (octave > 7) octave = 7;
+    if (octave > 8) octave = 8;   // parser accepts octaves 4-8; B8 (~7.9 kHz) is within range
     uint32_t f = (uint32_t)CHROM4[idx] << (octave - 4);
     return (uint16_t)(f > 25000 ? 25000 : f);
 }