smoke: route through teacher_pool so backward/projection paths fire

Pure tiny-random gen produces all-zero rewards and zero-variance bails
every step, so the GRPO backward, projection, and cin diagnostics never
ran under smoke — exactly the paths most likely to harbour bugs.

Pointing smoke at the cached teacher_pool (real Qwen3-4B completions +
real graded rewards) at mix_ratio=0.5 guarantees within-group reward
spread on every step. Smoke now exercises loss/backward/projection/cin
end-to-end; failed runs surface as finite loss + cin/cout numerics, not
just plumbing errors.

Side fix: decouple pool from prompt tokenization. Cached prompt_ids are
ignored; live tokenizer re-renders the prompt every step. Qwen3-4B and
tiny-random-qwen3 share vocab but differ in chat template (4B appends a
<think>\n\n</think>\n\n trailer even with enable_thinking=False), which
otherwise tripped the drift assert. Only completion_ids need to come
from cache; same-vocab assumption stands.

Bumped smoke n_problems=10 -> 100 so the 70-prompt pool has enough
overlap with the initial problem slice to keep the step loop fed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/projected_grpo/train.py

@@ -124,7 +124,7 @@ PRESETS: dict[str, dict] = {
     # That catches checkpoint-save bugs that only manifest after step 25 (e.g.
     # closure-scope NameErrors in the save path).
     "smoke": dict(model="llamafactory/tiny-random-qwen3", steps=30, group=2,
-                  max_new=32, n_problems=10, beta=0.0, prompts_per_step=1),
+                  max_new=32, n_problems=100, beta=0.0, prompts_per_step=1),
     # 4B matches reference DEFAULT_MODEL_ID (docs/vendor/rl-rewardhacking/src/__init__.py).
     # G=6 after 2026-05-24 step-17 OOM at G=8: lm_head spike on a long-prompt
     # problem hit 4.16 GiB / 2.5 GiB free. `logits_to_keep` cuts lm_head ~33%;
@@ -661,7 +661,6 @@ table columns:
             "resolved": json.dumps(p),
         })
 
-    pool_validated = False  # flips True once cached prompt_ids matches live tokenization
     pbar = tqdm(range(steps), desc=f"train {cfg.arm} {cfg.preset.value}", mininterval=60)
     for step in pbar:
         t0 = time.time()
@@ -730,27 +729,18 @@ table columns:
                 if len(pool_rows) < G_t:
                     idxs = idxs + torch.randint(0, len(pool_rows), (G_t - len(pool_rows),), generator=rng).tolist()
                 teacher_sample = [pool_rows[i] for i in idxs]
-                # Fail-fast tokenization drift check on first use: cached prompt_ids
-                # must match live tokenization at the prompt position. If this trips
-                # the pool was generated with a different tokenizer / chat template.
-                if not pool_validated:
-                    cached_ids = teacher_sample[0]["prompt_ids"][: int(teacher_sample[0]["plen"])]
-                    live_ids = enc.input_ids[0].tolist()
-                    if cached_ids != live_ids:
-                        raise ValueError(
-                            f"teacher pool tokenization drift on problem_id={prob['problem_id']}: "
-                            f"cached prompt_ids[:plen]={cached_ids[:12]}... vs "
-                            f"live enc={live_ids[:12]}... (lengths {len(cached_ids)} vs {len(live_ids)})"
-                        )
-                    pool_validated = True
                 # Student live-gen. gen_cfg.num_return_sequences is baked to G_s
                 # at construction (pool path) or = group (no-pool path).
                 with torch.no_grad():
                     out_s = model.generate(**enc, generation_config=gen_cfg).detach()
-                # Build teacher tensor: each cached row is plen + L_t_i; right-pad
-                # to common L within the teacher batch, then F.pad to match student L.
+                # Build teacher tensor: live-tokenized prompt + cached completion.
+                # Cached prompt_ids are ignored — re-tokenizing live makes the pool
+                # robust to chat-template / tokenizer drift between the model used
+                # for pool generation (Qwen3-4B) and the current student (e.g.
+                # tiny-random-qwen3 under smoke). Same vocab is assumed.
+                live_prompt_ids = enc.input_ids[0].tolist()
                 teacher_seqs = [
-                    torch.tensor(r["prompt_ids"] + r["completion_ids"], dtype=torch.long, device=device)
+                    torch.tensor(live_prompt_ids + r["completion_ids"], dtype=torch.long, device=device)
                     for r in teacher_sample
                 ]
                 L_t = max(s.shape[0] for s in teacher_seqs)