feat(mesh): Meshtastic RSSI/SNR + peer-location map wiring (backlog #14/#15, part 1)

Backend: parse_mesh_packet now decodes MeshPacket.rx_snr (field 8, float) and
rx_rssi (field 12, int32), and a new POSITION_APP branch decodes Position.
latitude_i/longitude_i (fields 1/2, sfixed32) -- all field numbers confirmed
against the canonical meshtastic/protobufs mesh.proto, not guessed. Threaded
through ParsedContact -> refresh_contacts -> MeshPeer (mirroring how
pkc_capable was wired for #17), so mesh.peers now surfaces real rssi/snr/lat/
lon instead of always-null. Fixed a real bug found along the way:
update_node_info's unconditional contact replace would have silently wiped
any already-tracked signal/position data on the next NodeInfo packet -- now
preserves it.

Frontend: mesh.ts's updateNodePositionsFromPeers() feeds real position data
into the SAME nodePositions map MeshMap.vue already renders from (parallel to
the existing Coordinate/Alert-message path) -- MeshMap.vue itself needed zero
changes, it was already built for this.

105/105 mesh tests pass (4 new: rx_snr/rx_rssi decode, position decode +
incomplete-field handling, full packet_to_inbound_frame integration).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/archipelago/src/mesh/protocol.rs

@@ -396,6 +396,16 @@ pub struct ParsedContact {
     /// NodeInfo public key, so the firmware delivers DMs PKC-encrypted). Meshcore
     /// contacts leave it `false` — their E2E status is tracked per-message.
     pub pkc_capable: bool,
+    /// Signal strength (dBm) / signal-to-noise ratio (dB) of the most recently
+    /// heard packet from this contact. Meshtastic-only today (from
+    /// `MeshPacket.rx_rssi`/`.rx_snr`); other transports leave these `None`.
+    pub rssi: Option<i16>,
+    pub snr: Option<f32>,
+    /// Last known position, from a Meshtastic `POSITION_APP` broadcast
+    /// (`Position.latitude_i`/`.longitude_i`, degrees). `None` until the
+    /// contact has shared one.
+    pub lat: Option<f64>,
+    pub lon: Option<f64>,
 }
 
 /// Parse RESP_CONTACT (0x03) response.
@@ -440,6 +450,13 @@ pub fn parse_contact(data: &[u8]) -> Result<ParsedContact> {
         flags,
         // Meshcore tracks E2E per message, not per contact.
         pkc_capable: false,
+        // Meshcore's own contact format does carry lat/lon at a fixed offset
+        // (see the format comment above) but wiring that up is out of scope
+        // for this Meshtastic-specific backlog item.
+        rssi: None,
+        snr: None,
+        lat: None,
+        lon: None,
     })
 }