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/JsonFormatter.cpp

@@ -16,6 +16,17 @@ void BufferedJsonFormatter::setBuffer(char *b, size_t sz) {
 size_t BufferedJsonFormatter::length() const {
   return this->_len;
 }
+void JsonFormatter::discard() {
+  // With no room every _safecat() takes the overflow path, and as it has
+  // already overflowed that doesn't log either.
+  this->buff = nullptr;
+  this->buffSize = 0;
+  this->_len = 0;
+  this->_nocomma = true;
+  this->_overflow = true;
+  this->_objects = 0;
+  this->_arrays = 0;
+}
 void JsonFormatter::beginObject(const char *name) {
   if(name && strlen(name) > 0) this->appendElem(name);
   else if(!this->_nocomma) this->_safecat(",");