fix: protocol 2 core review findings

From the review of #99:
- A sync could stall when a client took a slot the moment another left:
  loop() used up the new client's sync on the old one. The slots move
  to ProtoSlots, a pure module with native tests, and loop() takes a
  slot's wants only for the client it read (takeWants).
- Deleting a group sends its shades, whose inGroup changed. Deleting a
  shade unlinks it from its groups and sends them, so a new shade given
  its id doesn't join them.
- next and sync with an id get an ok reply, and a message in pieces
  gets one bad request, not one per piece.
- Feeds check for loop() before they write the event buffer, and log
  an event too large, as events do.
- editChanges() (ModelEditor, native tests) says what clients get after
  an edit; the edit result carries the shade a group gained or lost.
  order() takes a sort_list_t.
- The frame writers move to ProtoFrame. Renames: pf to proto_field_t,
  the X-macro letters to TYPE/FIELD/ERR/FEED, proto_request_t to
  proto_message_t, REQ_* to WANTS_*, fmt to eventJson. The event
  bodies share sendItem(), the connection type name lives in
  ESPNetwork, and the loop() task check in SocketEmitter.
- e2e: the heap floor check skips when the low since boot is already
  under it; tests for replies to next and sync and a long message.
- SECURITY_PLAN.md splits step 1 into 1a-1d.

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

test/e2e/test_messages.py

@@ -35,6 +35,34 @@ def test_an_event_type_from_a_client_is_unknown(connect, schema):
     assert error_name(schema, reply_to(c, msg_id)) == "unknownOp"
 
 
+def test_next_and_sync_with_an_id_are_answered(connect, schema):
+    """A message with an id gets one reply, control messages too."""
+    c = synced(connect, schema)
+    msg_id = c.send(schema.type_numbers["next"], with_id=True)
+    assert reply_to(c, msg_id)["d"].get(schema.keys["ok"]) == 1
+    msg_id = c.send(schema.type_numbers["sync"], with_id=True)
+    assert reply_to(c, msg_id)["d"].get(schema.keys["ok"]) == 1
+
+
+def test_a_long_message_gets_one_reply(connect, schema):
+    """A frame bigger than a TCP segment reaches the device in pieces, which it
+    can't put together yet: still one reply for the whole message.  The message
+    is wrong either way, so a reply is due however it arrives."""
+    c = synced(connect, schema)
+    c.send_text('{"v":2,"t":%d,"id":5,"x":"%s"}' % (schema.type_numbers["shadeState"], "a" * 3000))
+    replies = []
+    try:
+        while True:
+            f = c.recv(timeout=3)
+            if f["t"] == 2:
+                replies.append(f)
+    except TimeoutError:
+        pass
+    assert len(replies) == 1, replies
+    # In pieces it can't be read, so its id isn't known; whole, it is an event.
+    assert error_name(schema, replies[0]) in ("badRequest", "unknownOp")
+
+
 def test_subscribing_to_a_feed_is_ok(connect, schema):
     c = synced(connect, schema)
     for feed in ("frames", "frequencyScan"):