feat(cfold): canonicalize custom test layout

tests/mumble/custom/test_welcome_text_roundtrip.py

@@ -0,0 +1,31 @@
+"""mumble — recipe-specific functional test #1 (Phase 2 P3, beyond parity).
+
+Config round-trip: the harness deploys the recipe with a unique WELCOME_TEXT marker
+(recipe_meta.EXTRA_ENV -> MUMBLE_CONFIG_WELCOMETEXT). A client that completes the handshake
+receives the server's welcome text in the ServerSync message. This proves our deploy-time
+configuration actually propagated into the running murmur server AND is delivered to clients over
+the real protocol — not just that a port is open. Version-independent (asserts OUR marker, not a
+hard-coded upstream string).
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+import _mumble_proto  # noqa: E402
+import recipe_meta  # noqa: E402
+
+
+def test_configured_welcome_text_surfaces_in_serversync(live_app):
+    marker = recipe_meta._WELCOME_TEXT_MARKER
+    r = _mumble_proto.retry_handshake(attempts=12, interval=5.0)
+
+    assert r["server_sync"], f"ServerSync handshake did not complete — {r.get('error')}"
+    welcome = r["welcome_text"] or ""
+    assert marker in welcome, (
+        f"configured welcome-text marker {marker!r} not present in the server's ServerSync "
+        f"welcome_text (got {welcome!r}) — deploy-time config did not propagate"
+    )