Unknown is a selectable property-type bucket 🟩

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/modelling/property_filters.py

@@ -74,6 +74,10 @@ def resolve_filtered_property_ids(
     ]
 
 
+# "Unknown" is itself a selectable filter value: exactly the bucket of
+# properties whose value resolves at no precedence level (ADR-0056).
+_UNKNOWN = "Unknown"
+
 # RdSAP numeric codes as lodged on some certs; text labels pass through as-is
 # (ADR-0056 — the mapping is part of the shared preview contract).
 _PROPERTY_TYPE_CODES: dict[str, str] = {
@@ -119,12 +123,11 @@ def _resolved_property_types(
 
     resolved: dict[int, str] = {}
     for pid in property_ids:
-        value = (
+        resolved[pid] = (
             by_override.get(pid)
             or by_epc_source["lodged"].get(pid)
             or by_epc_source["predicted"].get(pid)
             or by_legacy_column.get(pid)
+            or _UNKNOWN
         )
-        if value is not None:
-            resolved[pid] = value
     return resolved