catch and correct possible bad values for lat/lon, add tests

lib/services/coordinate_validation.dart

@@ -0,0 +1,23 @@
+/// Shared validation helpers for geographic coordinates and zoom levels.
+///
+/// Used anywhere a [LatLng]-like value or zoom level comes from an external
+/// or platform source (GPS fixes, persisted preferences, live map camera
+/// state) that isn't guaranteed to be finite/in-range. Centralizing this
+/// avoids subtly inconsistent validation logic being duplicated across the
+/// GPS controller, map position persistence, and live camera guards.
+library;
+
+/// Validate that a latitude or longitude value is finite and within the
+/// valid range for geographic coordinates (-180 to 180).
+///
+/// Note: this intentionally uses the wider +/-180 range (rather than +/-90
+/// for latitude) so a single helper can validate both latitude and
+/// longitude values without the caller needing to track which is which.
+bool isValidCoordinate(double value) {
+  return !value.isNaN && !value.isInfinite && value >= -180.0 && value <= 180.0;
+}
+
+/// Validate that a zoom level is finite and within a sane range.
+bool isValidZoom(double zoom, {double min = 0.0, double max = 25.0}) {
+  return !zoom.isNaN && !zoom.isInfinite && zoom >= min && zoom <= max;
+}