Fix channel calculation for NARROW_868 region

Correct the `numChannels` and `radioFreq` calculations to align with the firmware logic. This primarily affects the NARROW_868 region by properly accounting for channel spacing.

- Remove `regionInfo.spacing` from the `numChannels` numerator.
- Add `regionInfo.spacing` to the `radioFreq` calculation.
- Add tests to verify `numChannels` and `radioFreq` for the `NARROW_868` region.

core/model/src/main/kotlin/org/meshtastic/core/model/ChannelOption.kt

@@ -78,10 +78,9 @@ val LoRaConfig.numChannels: Int
         if (bw <= 0f) return 1 // Return 1 if bandwidth is zero or negative
 
         // Calculate number of channels: spacing = gap between channels (0 for continuous spectrum)
-        // Match firmware: uint32_t numChannels = round((myRegion->freqEnd - myRegion->freqStart + myRegion->spacing) /
-        // channelSpacing);
+        // Match firmware: uint32_t numChannels = round((myRegion->freqEnd - myRegion->freqStart) / channelSpacing);
         val channelSpacing = regionInfo.spacing + bw
-        val num = Math.round((regionInfo.freqEnd - regionInfo.freqStart + regionInfo.spacing) / channelSpacing).toInt()
+        val num = Math.round((regionInfo.freqEnd - regionInfo.freqStart) / channelSpacing).toInt()
 
         // If the regional frequency range is smaller than the bandwidth, the firmware would
         // fall back to a default preset. In the app, we return 1 to avoid a crash.
@@ -100,13 +99,14 @@ internal fun LoRaConfig.radioFreq(channelNum: Int): Float {
     return if (regionInfo != null) {
         val bw = bandwidth(regionInfo)
         val channelSpacing = regionInfo.spacing + bw
-        // Match firmware: float freq = myRegion->freqStart + (bw / 2000) + (channel_num * channelSpacing);
+        // Match firmware: float freq = myRegion->freqStart + myRegion->spacing + (bw / 2000) + (channel_num *
+        // channelSpacing);
         // Note: firmware channel_num is 0-indexed in the calculation, but the app uses 1-indexed for some reason?
         // Let's re-verify the firmware logic.
         // Firmware: channel_num = hash(channelName) % numChannels;
-        // freq = myRegion->freqStart + (bw / 2000) + (channel_num * channelSpacing);
+        // freq = myRegion->freqStart + myRegion->spacing + (bw / 2000) + (channel_num * channelSpacing);
         // The app's channelNum function returns 1..numChannels if channelNum is 0.
-        (regionInfo.freqStart + bw / 2) + (channelNum - 1) * channelSpacing
+        (regionInfo.freqStart + regionInfo.spacing + bw / 2) + (channelNum - 1) * channelSpacing
     } else {
         0f
     }

core/model/src/test/kotlin/org/meshtastic/core/model/ChannelOptionTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025 Meshtastic LLC
+ * Copyright (c) 2025-2026 Meshtastic LLC
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -14,13 +14,14 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-
 package org.meshtastic.core.model
 
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNotNull
 import org.junit.Test
+import org.meshtastic.proto.ConfigKt.loRaConfig
 import org.meshtastic.proto.ConfigProtos.Config.LoRaConfig.ModemPreset
+import org.meshtastic.proto.ConfigProtos.Config.LoRaConfig.RegionCode
 
 class ChannelOptionTest {
 
@@ -78,4 +79,24 @@ class ChannelOptionTest {
             ChannelOption.entries.size,
         )
     }
+
+    @Test
+    fun `test radioFreq and numChannels for NARROW_868`() {
+        val loraConfig = loRaConfig {
+            region = RegionCode.NARROW_868
+            usePreset = true
+            modemPreset = ModemPreset.NARROW_FAST
+        }
+
+        // bw = 0.0625, spacing = 0.015, channelSpacing = 0.0775
+        // Range = 869.65 - 869.4 = 0.25
+        // numChannels = round(0.25 / 0.0775) = 3
+        assertEquals(3, loraConfig.numChannels)
+
+        // Slot 1: freqStart + spacing + bw/2 = 869.4 + 0.015 + 0.03125 = 869.44625
+        assertEquals(869.44625f, loraConfig.radioFreq(1), 0.0001f)
+
+        // Slot 3: 869.44625 + 2 * 0.0775 = 869.44625 + 0.155 = 869.60125
+        assertEquals(869.60125f, loraConfig.radioFreq(3), 0.0001f)
+    }
 }