feat(mqtt): add neighbors JSON payload builder (host-tested)

Add buildNeighborsMessage to the pure MQTTPayloadBuilder core and a thin
delegating wrapper + NeighborsMessageEntry alias on MQTTMessageBuilder, so
the neighbors topic is built by the same firmware-facing API as status/
packet/raw while the layout logic stays exercisable by native tests.

The document is bounded to the publish buffer: entries arrive ordered most-
to least-useful and the tail is dropped once the next entry would overflow,
so a fixed PSRAM buffer can never be handed truncated JSON.

Uses ArduinoJson v7 idioms (.to<JsonObject>()/.add<JsonObject>()) to stay
warning-clean under -Werror, unlike the deprecated createNested* forms.

Adds three test_mqtt_payload_builder cases: self+entry round-trip, empty
table / null scopes, and bounded-growth tail-drop under a tight buffer.

src/helpers/MQTTMessageBuilder.cpp

@@ -102,6 +102,22 @@ int MQTTMessageBuilder::buildRawMessage(
       origin, origin_id, timestamp, raw, buffer, buffer_size);
 }
 
+int MQTTMessageBuilder::buildNeighborsMessage(
+  JsonDocument& doc,
+  const char* origin,
+  const char* origin_id,
+  const char* timestamp,
+  const char* self_scopes,
+  const NeighborsMessageEntry* neighbors,
+  int neighbor_count,
+  char* buffer,
+  size_t buffer_size
+) {
+  return MQTTPayloadBuilder::buildNeighborsMessage(
+      doc, origin, origin_id, timestamp, self_scopes, neighbors, neighbor_count,
+      buffer, buffer_size);
+}
+
 int MQTTMessageBuilder::buildPacketJSON(
   JsonDocument& doc,
   mesh::Packet* packet,

src/helpers/MQTTMessageBuilder.h

@@ -2,6 +2,7 @@
 
 #include "MeshCore.h"
 #include <ArduinoJson.h>
+#include "MQTTPayloadBuilder.h"
 #include <Mesh.h>
 #include <Timezone.h>
 
@@ -153,6 +154,22 @@ public:
     size_t buffer_size
   );
 
+  // Neighbors table entry + JSON builder. The layout logic lives in the pure,
+  // host-tested MQTTPayloadBuilder; this is the firmware-facing alias/delegate,
+  // matching the status/packet/raw builders.
+  using NeighborsMessageEntry = MQTTPayloadBuilder::NeighborsMessageEntry;
+  static int buildNeighborsMessage(
+    JsonDocument& doc,
+    const char* origin,
+    const char* origin_id,
+    const char* timestamp,
+    const char* self_scopes,
+    const NeighborsMessageEntry* neighbors,
+    int neighbor_count,
+    char* buffer,
+    size_t buffer_size
+  );
+
   /**
    * Convert packet to JSON message
    *

src/helpers/MQTTPayloadBuilder.cpp

@@ -184,3 +184,48 @@ int MQTTPayloadBuilder::buildRawMessage(
 
   return serializeComplete(root, buffer, buffer_size);
 }
+
+int MQTTPayloadBuilder::buildNeighborsMessage(
+  JsonDocument& doc,
+  const char* origin,
+  const char* origin_id,
+  const char* timestamp,
+  const char* self_scopes,
+  const NeighborsMessageEntry* neighbors,
+  int neighbor_count,
+  char* buffer,
+  size_t buffer_size
+) {
+  if (!buffer || buffer_size == 0) return 0;
+
+  doc.clear();
+  JsonObject root = doc.to<JsonObject>();
+  root["timestamp"] = timestamp;
+  root["origin"] = origin;
+  root["origin_id"] = origin_id;
+
+  JsonObject self = root["self"].to<JsonObject>();
+  self["scopes"] = self_scopes ? self_scopes : "";
+
+  JsonArray arr = root["neighbors"].to<JsonArray>();
+  if (measureJson(root) >= buffer_size) return 0;
+
+  for (int i = 0; i < neighbor_count; i++) {
+    JsonObject nb = arr.add<JsonObject>();
+    nb["pubkey"] = neighbors[i].pubkey_hex;
+    nb["snr"] = neighbors[i].snr;
+    nb["heard_secs_ago"] = neighbors[i].heard_secs_ago;
+    nb["scopes"] = neighbors[i].scopes ? neighbors[i].scopes : "";
+    nb["status"] = neighbors[i].status;
+
+    // Entries arrive ordered most- to least-useful. Stop as soon as the next
+    // one would fill the fixed publish buffer, dropping the remaining tail so
+    // document growth stays bounded.
+    if (measureJson(root) >= buffer_size) {
+      arr.remove(arr.size() - 1);
+      break;
+    }
+  }
+
+  return serializeComplete(root, buffer, buffer_size);
+}

src/helpers/MQTTPayloadBuilder.h

@@ -68,5 +68,28 @@ public:
     char* buffer,
     size_t buffer_size
   );
+
+  struct NeighborsMessageEntry {
+    const char* pubkey_hex;
+    float snr;
+    uint32_t heard_secs_ago;
+    const char* scopes;
+    const char* status;
+  };
+
+  // Build neighbors-table JSON for the meshcore/{iata}/{device}/neighbors topic.
+  // Callers order entries most- to least-useful; document growth is bounded to
+  // buffer_size and the remaining tail is dropped once the next entry won't fit.
+  static int buildNeighborsMessage(
+    JsonDocument& doc,
+    const char* origin,
+    const char* origin_id,
+    const char* timestamp,
+    const char* self_scopes,
+    const NeighborsMessageEntry* neighbors,
+    int neighbor_count,
+    char* buffer,
+    size_t buffer_size
+  );
 };