feat(content-build): validation gate (rungs + resolve)

tools/content_build/rungs.py

@@ -0,0 +1,23 @@
+"""The rung vocabulary, sourced from the bible's disposition-ladder entry. The
+bible owns rung names/order; band NUMBERS live in client code and never here."""
+
+from .errors import BuildError
+from .model import Entry
+
+LADDER_ID = "rule.disposition-ladder"
+NEVER = "never"
+
+
+def ladder_rungs(entries: list[Entry]) -> list[str]:
+    ladders = [e for e in entries if e.id == LADDER_ID]
+    if len(ladders) != 1:
+        raise BuildError(
+            f"exactly one '{LADDER_ID}' entry required, found {len(ladders)}")
+    if not ladders[0].rungs:
+        raise BuildError("disposition-ladder needs a non-empty 'rungs:' list",
+                         source=ladders[0].source, entry_id=LADDER_ID)
+    return list(ladders[0].rungs)
+
+
+def legal_gates(entries: list[Entry]) -> set[str]:
+    return set(ladder_rungs(entries)) | {NEVER}