Track B: held-out data registry for every Track B and open-population gate

scripts/first_estimates_birth_evidence.py

@@ -324,6 +324,10 @@
     # by B2's floor builder, comparator and scorer; nothing in the
     # historical projection imports it.
     Path("src/populace_dynamics/track_b/b2.py"),
+    # The held-out data registry's checks read only the registry, git and
+    # committed run artifacts; nothing in the historical projection
+    # imports them.
+    Path("src/populace_dynamics/track_b/holdout_registry.py"),
     # S1 development and future registration; no historical projection imports.
     Path("src/populace_dynamics/engine/forward_earnings_s1_v1.py"),
     # U2 documentary metadata stays outside the historical projection.