fix: xref member-mark regex covers single-letter marks; recheck sheet diversity after cap

backend/agents/linker.py

@@ -38,7 +38,7 @@ def _xref_keys(assertion: Dict) -> List[str]:
     if ref:
         keys.append(f"detail:{ref}")
     tag = re.sub(r"\s+", "", str(location.get("tag") or "")).upper()
-    if re.match(r"^[A-Z]{2,}\d", tag):  # member marks: HSS16X4X5/8, W12X26, ...
+    if re.match(r"^[A-Z]+\d", tag):  # member marks: W12X26, HSS16X4X5/8, ...
         keys.append(f"tag:{tag}")
     return keys
 
@@ -76,9 +76,12 @@ def build_link_scopes(sheets: List[Dict]) -> List[AgentScope]:
         sheets_present = {a.get("sheet_number") for a in assertions}
         if len(assertions) < 2 or len(sheets_present) < 2:
             continue
+        chunk = assertions[:cap]
+        if len({a.get("sheet_number") for a in chunk}) < 2:
+            continue  # cap landed on a single sheet — xref adds nothing
         scopes.append(AgentScope(
             scope_id=f"xref:{key}",
-            payload={"assertions": assertions[:cap],
+            payload={"assertions": chunk,
                      "level": "xref", "family": key},
         ))
     return scopes