fix(qth): correct Maidenhead boundaries and longitude wrapping

A full-domain round-trip probe found three related boundary bugs.

1. Exact positive limits wrapped the square/subsquare terms to zero

positionToQth clamped only the A-R field index. At +90 latitude / +180
longitude the field saturated at R, but all later terms used modulo and wrapped
to square 0 / subsquare a:

  (90, 180) -> RR00aa00 -> (80.002083, 160.004167)
  error: -9.998 deg latitude, -19.996 deg longitude

The existing test incorrectly asserted RR00aa00 and had therefore fossilised
the defect. Clamp shifted coordinates just inside the half-open upper bound so
the limits land in the final cell RR99xx99.

2. isValidPosition allowed longitude through +360

Maidenhead covers -180..180, but 181..360 was accepted and produced plausible
locators that decoded 20-200 degrees away:

  lon 181 -> decoded 161.004167  (error -19.996)
  lon 270 -> decoded 170.004167  (error -99.996)
  lon 360 -> decoded 160.004167  (error -199.996)

Restrict the converter contract to -180..180.

3. Locator validation allowed S-X as field letters

The first pair has 18 fields A-R, while only the later subsquare pairs use
A-X. The shared [A-X]{2} regex accepted SS00aa / XX99xx and decoded them past
the poles (up to lat 149.98, lon 299.96). Use A-R for the field pair.

The SettingsRepo caller had a separate wrapping bug that masked part of this:
it mapped longitude>180 by subtracting 180 (270 -> +90, wrong hemisphere)
instead of modulo 360 (270 -> -90). Fix that at the writer too.

Verification:
- standalone JVM sweep: 519,841 points, old code had 1,441 large-error points
  with max drift 9.997917 deg lat / 19.995833 deg lon
- new Kotlin regression sweep requires every 8-char round trip <=0.01 deg
- QthConverterTest BUILD SUCCESSFUL
- full :core:domain:test + :core:data:compileReleaseKotlin BUILD SUCCESSFUL

core/data/src/main/java/com/rtbishop/look4sat/core/data/repository/SettingsRepo.kt

@@ -181,7 +181,10 @@ class SettingsRepo(
     }
 
     override fun setStationPosition(latitude: Double, longitude: Double, altitude: Double): Boolean {
-        val newLongitude = if (longitude > 180.0) longitude - 180 else longitude
+        // Wrap an out-of-range longitude into -180..180. Subtracting 180 (the
+        // previous behaviour) mapped 270 to +90 instead of -90, i.e. the wrong
+        // hemisphere, and 360 to +180 instead of 0.
+        val newLongitude = ((longitude + 180.0).mod(360.0)) - 180.0
         val locator = positionToQth(latitude, newLongitude) ?: return false
         setStationPosition(latitude, newLongitude, altitude, locator)
         return true