fix(map): polar field-row grid lines vanish at field zoom

Maidenhead row R (80-90N) and row A (-90..-80S) vertical lines were drawn
with projectionToY(±90). Web Mercator diverges there: osmdroid's
TileSystemWebMercator yields y01=±Infinity -> Long.MIN/MAX_VALUE pixel
coordinates (verified via JVM test: lat=90 -> -9.2e18, lat=-90 -> +9.2e18).
The canvas cannot rasterize such coordinates, so the polar rows' grid
lines silently disappeared at field zoom (2-char labels).

Fix: clamp the projected latitude to the Mercator limit ±85.0511 in
projectionToY(). The polar rows then project onto the screen edge and
their lines render (the sliver beyond 85.05 is off-screen anyway). Row A
kept its symmetric handling; row R now uses the same path.

feature/map/src/main/java/com/rtbishop/look4sat/feature/map/MaidenheadGridOverlay.kt

@@ -428,7 +428,15 @@ class MaidenheadGridOverlay : Overlay() {
 
     /** Y pixel for a latitude, or null when outside the viewport. */
     private fun projectionToY(projection: Projection, lat: Double): Float? {
-        val geo = org.osmdroid.util.GeoPoint(lat, 0.0)
+        // Web Mercator is undefined beyond ±85.0511° (osmdroid clamps the world
+        // to TileSystemWebMercator limits). Projecting the polar field rows
+        // (lat=±90: Maidenhead row R = 80..90°N, row A = -90..-80°S) yields
+        // ±9.2e18 pixel coordinates that the canvas cannot rasterize, so the
+        // polar grid lines silently vanish. Clamp to the Mercator limit: the
+        // polar rows then project onto the screen edge and their lines are
+        // drawn (the off-limit sliver beyond 85.05° is invisible anyway).
+        val clamped = lat.coerceIn(-MAX_MERCATOR_LAT, MAX_MERCATOR_LAT)
+        val geo = org.osmdroid.util.GeoPoint(clamped, 0.0)
         val p = projection.toPixels(geo, null)
         return p.y.toFloat()
     }
@@ -484,6 +492,9 @@ class MaidenheadGridOverlay : Overlay() {
         const val FIELD_LON = 20.0
         const val SUB_SQUARE_LAT = 1.0
         const val SUB_SQUARE_LON = 2.0
+        // Web Mercator latitude limit (osmdroid TileSystemWebMercator): projecting
+        // beyond it produces ±9.2e18 pixel coordinates the canvas cannot draw.
+        const val MAX_MERCATOR_LAT = 85.05112877980658
         // Zoom at which the 2°x1° sub-square grid lines/fills appear.
         // 5.0 → 6.0: at zoom 5 a full field spans too little screen width and
         // the sub-square grid is too dense to read; 6 roughly doubles the