fix(retrieval): declare the reply moment's surfacing site in FAN_OUT_SITES (milestone 458 step 4b, #4922)

moment_delivery records the mounted half's surfacing under
source=rp.MOMENT_RULE_SOURCE. That is an attribute the registry's
extractor cannot resolve, so the site is now declared with the value it
emits, read from the pipeline's own constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/retrieval_registry.py

@@ -42,7 +42,7 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 
-from scribe.services.retrieval_pipeline import RULE_ARMS, RULE_SOURCES
+from scribe.services.retrieval_pipeline import MOMENT_RULE_SOURCE, RULE_ARMS, RULE_SOURCES
 
 # ── How a point is reached ────────────────────────────────────────────────
 #
@@ -282,6 +282,12 @@ FAN_OUT_SITES: dict[str, tuple[str, ...]] = {
     "scribe/services/retrieval_pipeline.py::record_rule_surfaced(source=source)": (
         tuple(arm.source for arm in RULE_ARMS)
     ),
+    # The reply moment's mounted half (milestone 458): it records under the
+    # moment arm's source, read from the pipeline's constant so the two doors
+    # cannot drift apart on the name.
+    "scribe/services/moment_delivery.py::record_rule_surfaced(source=<expression>)": (
+        MOMENT_RULE_SOURCE,
+    ),
 }