docs: add M9N GPS module plan

docs/gps-m9n.md

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+# M9N GPS Module Plan
+
+## Role
+
+The M9N GPS module will live in the dashboard enclosure and provide:
+
+- GPS position
+- GPS speed
+- Satellite/fix status
+- UTC time
+- Future automatic timezone/DST support
+- Optional GPS fallback speed display
+
+## Location
+
+Preferred mounting location:
+
+- Inside the top dashboard enclosure
+- Under the windshield
+- Near the ESP32-S3 dashboard display
+
+This should provide a clean enough sky view for normal vehicle GPS use.
+
+## Wiring
+
+Planned connection to dashboard ESP32-S3:
+
+- GPS TX -> ESP32 UART RX
+- GPS RX -> ESP32 UART TX, optional
+- GPS VCC -> 3.3V or 5V depending on module board
+- GPS GND -> dashboard ground
+
+PPS is optional and not required for v1.
+
+## V1 Data Goals
+
+Initial GPS fields:
+
+- Fix valid
+- Satellite count
+- Latitude
+- Longitude
+- Speed
+- UTC time
+
+## Future Features
+
+- Automatic timezone lookup
+- Automatic DST handling
+- Trip logging
+- GPS-based speed fallback
+- GPS diagnostics screen