feat(client): DmService narrate loop + FakeDmTransport (all branches tested)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/tests/doubles/fake_transport.gd

@@ -0,0 +1,19 @@
+extends "res://scripts/net/dm_transport.gd"
+## Test double for DmTransport. Returns a canned DmResponse and records the last
+## call so tests can assert what the service posted. post_json needs no real
+## async — awaiting a plain return value resolves immediately, so the same await
+## call site in DmService drives both the real and the fake transport.
+
+var _canned
+var last_path: String
+var last_body: Dictionary
+
+
+func set_response(resp) -> void:
+	_canned = resp
+
+
+func post_json(path: String, body: Dictionary) -> DmResponse:
+	last_path = path
+	last_body = body
+	return _canned