M2 step 6: Tracks, recorded to GPX in the background

`r` in the GNSS App (or `gnss track start|stop`) records a Track to
/gnss/tracks/YYYYMMDD-HHMMSS.gpx: a point every 5 s once moved 5 m
(lib/gnss/track, 7 tests: haversine, the rule, GPX text, the file name).
It keeps recording with the App closed, shows REC in the Status Bar, and
announces start and stop with a Toast. It needs a card and the time;
switching GNSS off stops it. Tracks are written like Captures: past 90 %
card usage, until full. Storage Clean-up gets a GNSS tracks category.

A Track cut short by a reset or power loss has no GPX footer; the GNSS
Service closes such files at the next boot.

Verified on the device: a recorded Track and one interrupted by a reset
both parse as GPX 1.1 on the PC.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/gnss/src/track.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace roro::gnss {
+
+// Great-circle distance in metres (haversine, mean Earth radius).
+double distanceMeters(double lat1, double lon1, double lat2, double lon2);
+
+// When a Track takes its next point (Q63): the first one always, then at least every 5 s and only
+// after moving at least 5 m, so standing still doesn't fill the card.
+class TrackRule {
+   public:
+    static constexpr uint32_t kEveryMs = 5000;
+    static constexpr double kMinMeters = 5.0;
+
+    // True if this position should be recorded (and is then remembered as the last point).
+    bool due(uint32_t nowMs, double lat, double lon);
+    void reset() { any_ = false; }
+
+   private:
+    bool any_ = false;
+    uint32_t lastMs_ = 0;
+    double lat_ = 0, lon_ = 0;
+};
+
+// "/gnss/tracks/YYYYMMDD-HHMMSS.gpx", from the UTC start time (Storage Clean-up reads its date).
+std::string trackPath(int64_t utcSeconds);
+
+// GPX 1.1, written a line at a time: the header when a Track starts, a point per line, the footer
+// when it stops. A file whose footer never came (power lost) is closed at the next boot.
+namespace gpx {
+std::string header(const std::string& name);
+std::string point(double lat, double lon, bool hasElevation, float elevationM, int64_t utcSeconds);
+std::string footer();
+bool finished(const std::string& tail);  // the end of a file: does it close the GPX?
+}  // namespace gpx
+
+}  // namespace roro::gnss