fix: center coverage bands on computed grid samples

feature/coverage/src/commonMain/kotlin/org/meshtastic/feature/coverage/CoverageContours.kt

@@ -38,21 +38,21 @@ internal fun CoverageGrid.bands(ranges: List<ClosedFloatingPointRange<Double>>):
         // Greedily merge horizontal runs of in-band cells into rectangles: far fewer polygons than
         // one per cell, and the map renders a handful of rings rather than thousands.
         val taken = BooleanArray(width * height)
-        for (y in 0 until height - 1) {
+        for (y in 0 until height) {
             var x = 0
-            while (x < width - 1) {
+            while (x < width) {
                 if (taken[y * width + x] || !inBand(x, y, range)) {
                     x++
                     continue
                 }
                 var runEnd = x
-                while (runEnd + 1 < width - 1 && !taken[y * width + runEnd + 1] && inBand(runEnd + 1, y, range)) {
+                while (runEnd + 1 < width && !taken[y * width + runEnd + 1] && inBand(runEnd + 1, y, range)) {
                     runEnd++
                 }
                 // Extend downwards while the whole run stays in band.
                 var runBottom = y
                 while (
-                    runBottom + 1 < height - 1 &&
+                    runBottom + 1 < height &&
                     (x..runEnd).all { inBand(it, runBottom + 1, range) } &&
                     (x..runEnd).none { taken[(runBottom + 1) * width + it] }
                 ) {
@@ -63,11 +63,11 @@ internal fun CoverageGrid.bands(ranges: List<ClosedFloatingPointRange<Double>>):
                 }
                 rings.add(
                     listOf(
-                        lonAt(x.toDouble()) to latAt(y.toDouble()),
-                        lonAt(runEnd + 1.0) to latAt(y.toDouble()),
-                        lonAt(runEnd + 1.0) to latAt(runBottom + 1.0),
-                        lonAt(x.toDouble()) to latAt(runBottom + 1.0),
-                        lonAt(x.toDouble()) to latAt(y.toDouble()),
+                        lonAt(x - HALF_CELL) to latAt(y - HALF_CELL),
+                        lonAt(runEnd + HALF_CELL) to latAt(y - HALF_CELL),
+                        lonAt(runEnd + HALF_CELL) to latAt(runBottom + HALF_CELL),
+                        lonAt(x - HALF_CELL) to latAt(runBottom + HALF_CELL),
+                        lonAt(x - HALF_CELL) to latAt(y - HALF_CELL),
                     ),
                 )
                 x = runEnd + 1
@@ -142,6 +142,7 @@ $features
 """
 }
 
+private const val HALF_CELL = 0.5
 private const val DEFAULT_BANDS = 6
 private const val EMPTY_FEATURE_COLLECTION =
     """{"type":"FeatureCollection","properties":{"generator":"meshtastic-kp1812"},"features":[]}"""

feature/coverage/src/commonTest/kotlin/org/meshtastic/feature/coverage/CoverageContoursTest.kt

@@ -17,6 +17,7 @@
 package org.meshtastic.feature.coverage
 
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertTrue
 
 class CoverageContoursTest {
@@ -48,4 +49,23 @@ class CoverageContoursTest {
 
         assertTrue("\"name\": \"Tom's \\\"Hill\\\" \\\\ relay\"" in geoJson, geoJson.take(300))
     }
+
+    @Test
+    fun mergedBandExtendsHalfACellBeyondEverySample() {
+        val grid =
+            CoverageGrid(
+                site = Site("Test", 10.0, 26.0, 915.0, 30.0, -130.0),
+                width = 2,
+                height = 2,
+                north = 12.0,
+                south = 8.0,
+                east = 32.0,
+                west = 20.0,
+                dbm = DoubleArray(4) { -90.0 },
+            )
+
+        val ring = grid.bands(listOf(-130.0..-80.0)).single().rings.single()
+
+        assertEquals(listOf(14.0 to 14.0, 38.0 to 14.0, 38.0 to 6.0, 14.0 to 6.0, 14.0 to 14.0), ring)
+    }
 }