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.h

@@ -24,6 +24,9 @@ class JsonFormatter {
     void _appendNumber(const char *name);
   public:
     bool overflowed() const { return _overflow; }
+    // Drops whatever is written until a buffer is set again.  Nothing is
+    // written anywhere, so tasks can share one discarding formatter.
+    void discard();
     // Writes c as it must appear inside a JSON string (out needs 7 bytes) and
     // returns its length.
     static size_t escapeChar(char c, char *out);