train_merge_bw: fall back to hard-only curriculum when easy pool is empty (BW direct pass ~1%)

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

scripts/train_merge_bw.py

@@ -93,14 +93,21 @@ def main():
     pool = {l: pool[l][12:] for l in pool}
     print(f"pool: easy={len(pool['easy'])} hard={len(pool['hard'])}",
           flush=True)
-    if min(len(pool["easy"]), len(pool["hard"])) < BATCH:
+    buckets = K_BUCKETS
+    if len(pool["easy"]) < BATCH:
+        # BW direct pass ~1%: easy bucket is empty by construction; train
+        # hard-only rather than aborting the arm
+        buckets = [(2, ("hard",)), (4, ("hard",))]
+        print(f"easy pool too small ({len(pool['easy'])}) — "
+              "hard-only curriculum k=2/4", flush=True)
+    if len(pool["hard"]) < BATCH:
         print("INSUFFICIENT POOL — aborting", flush=True)
         return
 
     log = []
     t0 = time.time()
     for step in range(STEPS):
-        k, labels = K_BUCKETS[step % len(K_BUCKETS)]
+        k, labels = buckets[step % len(buckets)]
         cand = [it for lbl in labels for it in pool[lbl]]
         batch = rng.sample(cand, BATCH)
         ids, msk, lab, lmask = build_batch(tok, batch)