fix: send socket events only from loop()

Every socket event is formatted into one static buffer by one shared
formatter.  setConnected() runs on the WiFi/Ethernet event task and sent
memStatus, wifiStrength and ethernet, then ran sockEmit.loop(), while
loop() could be building its own event.  _safecat() checks the room and
then writes at _len, so another task moving _len in between could write
past the end of g_response.

setConnected() now sends nothing.  loop() broadcasts the status every
STATUS_EMIT_MS (30 s): memStatus, then ethernet, or wifiStrength when it
moved 3 dBm or changed channel.  A client that connects still gets all
of it at once from initClients(), so needsBroadcast and the 1.5 s tick
go, and so does the 1.5 s memStatus for the config panel.

SocketEmitter records loop()'s task in begin().  Off it, beginEmit()
logs and hands out a formatter that writes nothing, and endEmit(),
endEmitRoom() and loop() do nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/outputs/SocketEmitter.h

@@ -22,6 +22,11 @@ class SocketEmitter {
   protected:
     uint8_t newclients = 0;
     uint8_t newClients[5] = {255,255,255,255,255};
+    // loop()'s task, recorded by begin().  Only it may send; see emitAllowed().
+    TaskHandle_t _owner = nullptr;
+    // What beginEmit() hands any other task, so the event goes nowhere.
+    JsonSockEvent _discard;
+    bool onLoopTask() const;
     void delayInit(uint8_t num);
   public:
     JsonSockEvent json;