fix: review fixes for step 4a

- e2e: the heap check uses a socket slot now, so it waits for the device
  to free it, and test_every_client_reconnecting_at_once closes its
  clients before the check runs.
- Comments and docs that still named removed things (the /controller read,
  the event buffer, memStatus, SocketEmitter, ModelSnapshot).
- ESPNetwork::emitSockets is emitStatus: one socket, and it sends the status.
- Unused includes, whitespace-only lines left by the toJSON removal.

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

test/test_utils/test_utils.cpp

@@ -232,7 +232,7 @@ static void test_emitAllowed_refuses_another_task() {
   TEST_ASSERT_FALSE(emitAllowed(&wifiEventTask, &loopTask));
 }
 static void test_emitAllowed_before_the_owner_is_known() {
-  // setup() before SocketEmitter::begin(), which is loop()'s task anyway.
+  // setup() before ProtoSocket::begin(), which is loop()'s task anyway.
   TEST_ASSERT_TRUE(emitAllowed(&loopTask, nullptr));
 }