fix(ui): give rx_snr real presence semantics end to end (#6523)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/common/src/commonMain/kotlin/org/meshtastic/core/common/util/MetricFormatter.kt

@@ -45,7 +45,12 @@ object MetricFormatter {
 
     fun pressure(hPa: Float, decimalPlaces: Int = 1): String = "${NumberFormatter.format(hPa, decimalPlaces)} hPa"
 
-    fun snr(value: Float, decimalPlaces: Int = 1): String = "${NumberFormatter.format(value, decimalPlaces)} dB"
+    /**
+     * Formats a signal-to-noise ratio, or [UNKNOWN_VALUE] when the packet carried no measurement. 0 dB is a legitimate
+     * reading, so it must never stand in for a missing one.
+     */
+    fun snr(value: Float?, decimalPlaces: Int = 1): String =
+        if (value == null) UNKNOWN_VALUE else "${NumberFormatter.format(value, decimalPlaces)} dB"
 
     /**
      * Formats a received signal strength, or [UNKNOWN_VALUE] when the radio reported none. 0 dBm is a legitimate

core/common/src/commonTest/kotlin/org/meshtastic/core/common/util/MetricFormatterTest.kt

@@ -74,6 +74,18 @@ class MetricFormatterTest {
     @Test
     fun snr() {
         assertEquals("5.5 dB", MetricFormatter.snr(5.5f))
+        assertEquals("-12.5 dB", MetricFormatter.snr(-12.5f))
+    }
+
+    @Test
+    fun snrAbsentIsUnknown() {
+        assertEquals("—", MetricFormatter.snr(null))
+    }
+
+    @Test
+    fun snrZeroIsARealReading() {
+        // Must not render as unknown: 0 dB is a signal at the noise floor, not a missing measurement.
+        assertEquals("0.0 dB", MetricFormatter.snr(0f))
     }
 
     @Test