fix: apply writes optimistically before the settle guard (issue #27)

mark_write_pending's settle window was dropping every update for a
just-written href, including the coordinator's own post-write refresh,
because nothing ever wrote the optimistic value into the cache for it
to protect. The write reflected on the device immediately but reverted
in HA until the next 30s summary sweep.

custom_components/localthings/coordinator.py

@@ -545,11 +545,21 @@ class LocalThingsCoordinator(DataUpdateCoordinator[dict[str, Any]]):
             return
         path_segs, body = result
 
+        # Apply the write optimistically before starting the settle guard,
+        # not after -- mark_write_pending gates every source (poll, sweep,
+        # observe) through the same apply(), itself included, so flipping
+        # this order would have the guard drop the one update it exists to
+        # protect. Without an optimistic value in the cache for it to hold
+        # onto, the settle window was just delaying the real device
+        # confirmation for a few seconds on every write, which read exactly
+        # like the write being silently reverted (issue #27).
+        self._observe.apply(href, body, source='optimistic')
+        self._observe.mark_write_pending(href)
+
         def _do_put():
             sess = self._session
             if sess is None:
                 raise RuntimeError("no session")
-            self._observe.mark_write_pending(href)
             code, _ = sess.post(path_segs, cbor2.dumps(body), timeout=8.0)
             self._log.info("PUT %s → code %#04x", href, code)