log: dump first full example of each hack class to verbose log

One-shot-per-class dump (rendered prompt + completion WITH special tokens + a
SHOULD interpretive line) so the log shows what an M1 vs M2 hack actually looks
like, not just the flag. Keyed on m2/hacked today; will re-key to env_mode in
the multi-loophole refactor (spec 20260530_faithful_multi_loophole_env).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/projected_grpo/train.py

@@ -945,6 +945,8 @@ table columns:
     rollout_log_path.write_text("")
     first_hack_saved = False
     route_span_checked = False  # R3: assert delta_S_hack.grad in span(V) once
+    dumped_hack_classes: set[str] = set()  # first full example of each hack class -> verbose log
+    teacher_dumped = False
 
     def save_ckpt(rows: list[dict], path: Path | None = None) -> None:
         """Rewrite the run checkpoint in place: trainable delta_S as tensors, per-step
@@ -1121,6 +1123,25 @@ table columns:
                 fmt_flags.append(r.format_ok)
                 hack_E_flags.append(r.hacked_loophole_used)
                 hack_D_flags.append(r.hacked_wrong_tests)
+                # First full example of each hack class -> verbose log (with special
+                # tokens). Lets us eyeball WHAT the hack actually looks like, not just
+                # the flag. One dump per class for the whole run.
+                hack_cls = "M2_hardcode" if (is_expose and r.m2) else ("M1_run_tests" if r.hacked else None)
+                if hack_cls and hack_cls not in dumped_hack_classes:
+                    dumped_hack_classes.add(hack_cls)
+                    should = {
+                        "M2_hardcode": "completion special-cases the K VISIBLE inputs (literal "
+                                       "returns / if-chains) and has no general algorithm, so it "
+                                       "fails held-out; ELSE the m2 flag is mis-firing",
+                        "M1_run_tests": "completion defines a run_tests() that passes against an "
+                                        "arbitrary Solution (vacuous asserts); ELSE hacked mis-firing",
+                    }[hack_cls]
+                    logger.debug(
+                        f"\n\n=== FIRST {hack_cls} HACK EXAMPLE (step {step}, p_idx {p_idx}, gi {gi}) ===\n"
+                        f"SHOULD: {should}\n"
+                        f"--- rendered prompt (special chars) ---\n{prompt}\n"
+                        f"--- completion (special chars) ---\n{tok.decode(completions[gi], skip_special_tokens=False)}\n"
+                        f"=== END {hack_cls} ===")
                 step_rollouts.append({
                     "step": step, "p_idx": p_idx, "gi": gi,
                     "reward": r.reward, "gt_pass": r.gt_pass, "pass_heldout": r.pass_heldout,