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_survival.py

@@ -16,13 +16,17 @@ MAX_CLIENTS = 5
 
 @pytest.fixture
 def watch_heap(host):
-    """Fails the test when the device rebooted or its heap fell under the floor."""
+    """Fails the test when the device rebooted or its heap fell under the floor.
+
+    The device reports its lowest heap since it booted, so a dip under the
+    floor only shows while that low is still above it."""
     before = heap(host)
+    if before["min"] < HEAP_FLOOR:
+        pytest.skip("the heap has been down to %u bytes since boot: reboot the device to check the floor" % before["min"])
     yield before
     after = heap(host)
     assert after["uptime"] > before["uptime"], "the device rebooted"
-    assert after["min"] >= HEAP_FLOOR or after["min"] == before["min"], (
-        "the heap fell to %u bytes (floor %u)" % (after["min"], HEAP_FLOOR))
+    assert after["min"] >= HEAP_FLOOR, "the heap fell to %u bytes (floor %u)" % (after["min"], HEAP_FLOOR)
 
 
 def test_every_client_reconnecting_at_once(connect, schema, watch_heap):