fix(messaging): carry absent snr through to the UI instead of 0 dB

The snrOrNull() seam stopped at the module boundary: MeshDataMapper narrowed an
absent reading to 0f because DataPacket.snr was not nullable, so MessageItem
rendered "SNR 0.00 dB" for a direct packet that carried no measurement -- a
reading the radio never took. Message.displayTime hides the equivalent problem
for time by falling back to received_time, but snr had no such guard.

Make the chain nullable end to end, following the template schema 51 already
set for rssi:

- DataPacket.snr, Message.snr, Reaction.snr, MeshBeaconOffer.snr -> Float?
- Room packet.snr and reactions.snr -> nullable, via AutoMigration(51 -> 52)
- MeshDataMapper passes snrOrNull() straight through, no fallback

As with the rssi migration, rows written before schema 52 keep their stored 0,
so a legacy 0 dB reading stays indistinguishable from "no reading" for existing
history; only new rows carry true presence. The column comments say so.

MeshBeaconInvitationCard's `offer.snr != 0f` check becomes a null check, which
also fixes its OR-gate showing "0 dB" as a real reading whenever rssi happened
to be present.

Left out deliberately: discovered_node.snr. Its readers aggregate --
DiscoveryRankingEngine takes a median over `nodes.map { it.snr }` and
DiscoveryMapViewModel dedups with `maxByOrNull { it.snr }` -- so nullability
there is a semantics decision (is an unmeasured node excluded from the median,
or sorted last?), not a mechanical change. It stays NOT NULL until that is
decided.

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

Author: jamesarich

core/database/schemas/org.meshtastic.core.database.MeshtasticDatabase/52.json

@@ -130,8 +130,9 @@ import org.meshtastic.core.database.entity.TracerouteNodePositionEntity
         AutoMigration(from = 48, to = 49),
         AutoMigration(from = 49, to = 50),
         AutoMigration(from = 50, to = 51),
+        AutoMigration(from = 51, to = 52),
     ],
-    version = 51,
+    version = 52,
     exportSchema = true,
 )
 @androidx.room3.ConstructedBy(MeshtasticDatabaseConstructor::class)

core/database/src/commonMain/kotlin/org/meshtastic/core/database/MeshtasticDatabase.kt

@@ -102,7 +102,8 @@ data class Packet(
     @ColumnInfo(name = "data") val data: DataPacket,
     @ColumnInfo(name = "packet_id", defaultValue = "0") val packetId: Int = 0,
     @ColumnInfo(name = "routing_error", defaultValue = "-1") var routingError: Int = -1,
-    @ColumnInfo(name = "snr", defaultValue = "0") val snr: Float = 0f,
+    /** Null when the packet carried no snr. Rows written before schema 52 store 0 for both absent and 0 dB. */
+    @ColumnInfo(name = "snr") val snr: Float? = null,
     /** Null when the radio reported no rssi. Rows written before schema 51 store 0 for both absent and 0 dBm. */
     @ColumnInfo(name = "rssi") val rssi: Int? = null,
     @ColumnInfo(name = "hopsAway", defaultValue = "-1") val hopsAway: Int = -1,
@@ -162,7 +163,8 @@ data class ReactionEntity(
     @ColumnInfo(name = "user_id") val userId: String,
     val emoji: String,
     val timestamp: Long,
-    @ColumnInfo(name = "snr", defaultValue = "0") val snr: Float = 0f,
+    /** Null when the packet carried no snr. Rows written before schema 52 store 0 for both absent and 0 dB. */
+    @ColumnInfo(name = "snr") val snr: Float? = null,
     /** Null when the radio reported no rssi. Rows written before schema 51 store 0 for both absent and 0 dBm. */
     @ColumnInfo(name = "rssi") val rssi: Int? = null,
     @ColumnInfo(name = "hopsAway", defaultValue = "-1") val hopsAway: Int = -1,

core/database/src/commonMain/kotlin/org/meshtastic/core/database/entity/Packet.kt

@@ -108,6 +108,29 @@ class MeshtasticDatabaseMigrationTest {
         }
     }
 
+    @Test
+    fun snrColumnsGoNullableWithoutLosingRows() = runTest {
+        helper.createDatabase(SNR_NULLABLE_FROM_VERSION).use { connection ->
+            connection.execSQL(
+                "INSERT INTO packet (uuid, myNodeNum, port_num, contact_key, received_time, read, data, snr, rssi) " +
+                    "VALUES (1, 42, 1, '0^all', 1000, 1, '{}', 0.0, -70)",
+            )
+            connection.execSQL(
+                "INSERT INTO reactions (myNodeNum, reply_id, user_id, emoji, timestamp, snr, rssi) " +
+                    "VALUES (42, 7, '!abc', 'X', 2000, -12.5, -70)",
+            )
+        }
+
+        helper.runMigrationsAndValidate(SNR_NULLABLE_TO_VERSION, emptyList()).use { connection ->
+            // A stored 0 dB must survive the recreate as 0, not become NULL: it is a real reading.
+            assertEquals(listOf("0.0"), queryColumn(connection, "SELECT snr FROM packet"))
+            assertEquals(listOf("-12.5"), queryColumn(connection, "SELECT snr FROM reactions"))
+            // A NULL is now storable where the column was previously NOT NULL DEFAULT 0.
+            connection.execSQL("UPDATE packet SET snr = NULL WHERE uuid = 1")
+            assertEquals(listOf(null), queryColumn(connection, "SELECT snr FROM packet"))
+        }
+    }
+
     /** Reads one column of every row as a string, with SQL NULL surfaced as Kotlin null. */
     private fun queryColumn(connection: SQLiteConnection, sql: String): List<String?> =
         connection.prepare(sql).use { statement ->
@@ -130,5 +153,7 @@ class MeshtasticDatabaseMigrationTest {
         const val EARLIEST_SCHEMA_VERSION = 3
         const val RSSI_NULLABLE_FROM_VERSION = 50
         const val RSSI_NULLABLE_TO_VERSION = 51
+        const val SNR_NULLABLE_FROM_VERSION = 51
+        const val SNR_NULLABLE_TO_VERSION = 52
     }
 }

core/database/src/jvmTest/kotlin/org/meshtastic/core/database/MeshtasticDatabaseMigrationTest.kt

@@ -51,7 +51,8 @@ data class DataPacket(
     var channel: Int = 0, // channel index
     var wantAck: Boolean = true, // If true, the receiver should send an ack back
     var hopStart: Int = 0,
-    var snr: Float = 0f,
+    /** Signal-to-noise ratio in dB, or null when the packet carried no measurement. 0 dB is a valid reading. */
+    var snr: Float? = null,
     /** Received signal strength, or null when the radio did not report one. 0 dBm is a valid reading. */
     var rssi: Int? = null,
     var replyId: Int? = null, // If this is a reply to a previous message, this is the ID of that message

core/model/src/commonMain/kotlin/org/meshtastic/core/model/DataPacket.kt

@@ -27,10 +27,15 @@ import org.meshtastic.proto.MeshBeacon
  *
  * @param fromNodeNum The node that broadcast the beacon (informational only — beacons are unsigned).
  * @param beacon The decoded advertisement, carrying the display [message][MeshBeacon.message] and the join offer.
- * @param snr Signal-to-noise ratio of the received beacon packet, in dB (0 when unknown).
+ * @param snr Signal-to-noise ratio of the received beacon packet, in dB, or null when the radio reported none.
  * @param rssi Received signal strength of the beacon packet, in dBm, or null when the radio reported none.
  */
-data class MeshBeaconOffer(val fromNodeNum: Int, val beacon: MeshBeacon, val snr: Float = 0f, val rssi: Int? = null) {
+data class MeshBeaconOffer(
+    val fromNodeNum: Int,
+    val beacon: MeshBeacon,
+    val snr: Float? = null,
+    val rssi: Int? = null,
+) {
     /** Stable identity for dedup/dismiss: a given sender advertising a given channel is one standing invitation. */
     val key: String
         get() = "$fromNodeNum:${beacon.offer_channel?.name.orEmpty()}"
@@ -55,9 +60,10 @@ data class MeshBeaconOffer(val fromNodeNum: Int, val beacon: MeshBeacon, val snr
 
         /**
          * Inverse of [encode]; returns null for a structurally malformed record (wrong field count, unparseable node
-         * number, or an undecodable beacon payload). An unparseable snr falls back to 0 and an unparseable rssi to
-         * absent — they are non-critical display metrics, not identity, so a bad numeric there does not discard an
-         * otherwise-valid invitation. An absent rssi encodes as `null`, which [String.toIntOrNull] round-trips back.
+         * number, or an undecodable beacon payload). An unparseable snr or rssi falls back to absent — they are
+         * non-critical display metrics, not identity, so a bad numeric there does not discard an otherwise-valid
+         * invitation. An absent value encodes as `null`, which [String.toFloatOrNull]/[String.toIntOrNull] round-trip
+         * back to null.
          */
         @Suppress("ReturnCount")
         fun decode(record: String): MeshBeaconOffer? {
@@ -66,7 +72,7 @@ data class MeshBeaconOffer(val fromNodeNum: Int, val beacon: MeshBeacon, val snr
             val node = parts[0].toIntOrNull() ?: return null
             val beaconBytes = parts.last().decodeBase64()?.toByteArray() ?: return null
             val beacon = runCatching { MeshBeacon.ADAPTER.decode(beaconBytes) }.getOrNull() ?: return null
-            return MeshBeaconOffer(node, beacon, parts[1].toFloatOrNull() ?: 0f, parts[2].toIntOrNull())
+            return MeshBeaconOffer(node, beacon, parts[1].toFloatOrNull(), parts[2].toIntOrNull())
         }
     }
 }

core/model/src/commonMain/kotlin/org/meshtastic/core/model/MeshBeaconOffer.kt

@@ -160,7 +160,8 @@ data class Message(
     val routingError: Int,
     val packetId: Int,
     val emojis: List<Reaction>,
-    val snr: Float,
+    /** Signal-to-noise ratio in dB, or null when the packet carried no measurement. 0 dB is a valid reading. */
+    val snr: Float?,
     /** Received signal strength, or null when the radio did not report one. 0 dBm is a valid reading. */
     val rssi: Int?,
     val hopsAway: Int,

core/model/src/commonMain/kotlin/org/meshtastic/core/model/Message.kt

@@ -24,7 +24,10 @@ data class Reaction(
     val user: User,
     val emoji: String,
     val timestamp: Long,
-    val snr: Float,
+    /**
+     * Signal-to-noise ratio in dB, or null when the packet carried no measurement (locally sent reactions included).
+     */
+    val snr: Float?,
     /** Received signal strength, or null when the radio did not report one (locally sent reactions included). */
     val rssi: Int?,
     val hopsAway: Int,

core/model/src/commonMain/kotlin/org/meshtastic/core/model/Reaction.kt

@@ -44,9 +44,7 @@ open class MeshDataMapper(private val nodeIdLookup: NodeIdLookup) {
             channel = if (packet.pki_encrypted == true) NodeAddress.PKC_CHANNEL_INDEX else packet.channel,
             wantAck = packet.want_ack == true,
             hopStart = packet.hop_start,
-            // Narrows absent to 0f because [DataPacket.snr] is not nullable; a genuine 0 dB reading and "no reading"
-            // become indistinguishable past this point. See [snrOrNull].
-            snr = packet.snrOrNull() ?: 0f,
+            snr = packet.snrOrNull(),
             rssi = packet.rx_rssi,
             replyId = decoded.reply_id,
             relayNode = packet.relay_node,

core/model/src/commonMain/kotlin/org/meshtastic/core/model/util/MeshDataMapper.kt

@@ -99,7 +99,7 @@ internal fun MeshBeaconInvitationCard(
                     color = MaterialTheme.colorScheme.onSurfaceVariant,
                 )
             }
-            if (offer.rssi != null || offer.snr != 0f) {
+            if (offer.rssi != null || offer.snr != null) {
                 Text(
                     text =
                     stringResource(