[api] Check the encoded end against the reserved size under ESPHOME_DEBUG_API

The fixed32 store helper moves to a private section since it neither bounds checks nor
advances the cursor, its comment describes the path each target takes, the generated
file scan flags any ProtoEncode call that does not assign the cursor, and StateWaiter
timeouts can carry a label so gathered waits are told apart.

tests/unit_tests/components/api/test_api_proto.py

@@ -387,6 +387,6 @@ def test_generated_encode_calls_keep_the_cursor() -> None:
     dropped = [
         line
         for line in CPP_TEXT.splitlines()
-        if line.lstrip().startswith("ProtoEncode::")
+        if "ProtoEncode::" in line and "pos = ProtoEncode::" not in line
     ]
     assert not dropped, dropped[:5]