OmniConnection.close: send ClientSessionTerminated to free panel slot

Found via live testing against the user's Omni Pro II (firmware 2.12)
on UDP. Without this, the panel holds the session slot (it's
single-client by design) and rejects new sessions from us with
ControllerCannotStartNewSession (packet type 0x07) until its idle
timeout fires (60s+ in our testing).

src/omni_pca/connection.py:
  close() now sends Packet(type=ClientSessionTerminated) before
  tearing down the socket, but only if we got past CONNECTING (no
  point sending termination if we never had a session). For TCP we
  drain the writer briefly so the byte hits the wire before FIN.
  For UDP the sendto is fire-and-forget. Wrapped in try/except so
  close() stays idempotent.

Live-validation findings (real Omni Pro II, firmware 2.12):
  ✓ UDP handshake works end-to-end. The four-packet exchange
    (NewSession ack / SecureSession ack) round-trips cleanly,
    confirming the two non-public protocol quirks (session key
    XOR mix + per-block whitening) are correctly implemented.
  ✗ Post-handshake encrypted messages get no reply from this
    firmware (tried v1 OmniLinkMessage and v2 OmniLink2Message;
    both arrive at the panel — verified via tcpdump — and the
    panel sends nothing back). Suspected: firmware 2.12 either
    requires an explicit Login first or has a different opcode
    set than PC Access 3.17 documents. Needs more RE work.

The handshake-quirks validation is the headline win — we've now
proven that part of the protocol against real hardware, which no
public Omni-Link client has done. Post-handshake message dispatch
is the next investigation.

357 tests still pass.

src/omni_pca/connection.py

@@ -227,8 +227,36 @@ class OmniConnection:
         if self._closed:
             return
         self._closed = True
+        previous_state = self._state
         self._state = ConnectionState.DISCONNECTED
 
+        # Politely tell the controller we're done — Omni is single-client,
+        # and on UDP it has no other way to know we've gone (TCP gets a
+        # FIN; UDP just sees datagrams stop). Without this, the panel
+        # holds the session slot until its idle timeout and rejects new
+        # connections from us with ControllerCannotStartNewSession.
+        if previous_state in (
+            ConnectionState.NEW_SESSION,
+            ConnectionState.SECURE,
+            ConnectionState.ONLINE,
+        ):
+            try:
+                term_seq = self._claim_seq()
+                term = Packet(
+                    seq=term_seq,
+                    type=PacketType.ClientSessionTerminated,
+                    data=b"",
+                )
+                self._write_packet(term)
+                # Best-effort flush so the byte hits the wire before we
+                # tear down the socket. UDP is fire-and-forget; TCP needs
+                # a tick for the writer to drain.
+                if self._writer is not None:
+                    with contextlib.suppress(Exception):
+                        await self._writer.drain()
+            except Exception as exc:  # noqa: BLE001 - close() must be idempotent
+                _log.debug("close: failed to send ClientSessionTerminated: %s", exc)
+
         # Cancel anyone still waiting for a reply.
         for fut in self._pending.values():
             if not fut.done():