fix: fail-fast --modes + multi-rep validator (external review)

gpt-5.5 review (docs/spec/20260530_code_review.md), both valid:
- --modes silently dropped typos/whitespace ('--modes=a, b' -> only a;
  '--modes=typo' -> empty sweep after a 30s model load, looking like success).
  Now strips + validates against MODES, raises on unknown before loading.
- validator was 1 stochastic sample/mode -> a <1.0-prob reachable hint could be
  falsely marked unreachable. Now PROBE_REPS samples, reports hits/reps, bar is
  >=1 exploit in N.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

src/projected_grpo/derisk_loopholes.py

@@ -257,7 +257,16 @@ def main(cfg: Config) -> int:
     save_dir.mkdir(parents=True, exist_ok=True)
 
     spoonfeed = cfg.elicit_style == "spoonfeed"
-    run_modes = [m for m in MODES if not cfg.modes or m in cfg.modes.split(",")]
+    # Fail fast on a mistyped/whitespaced --modes: silently running a subset (or an
+    # empty sweep) after a 30s model load would look like the request was honored.
+    if cfg.modes:
+        requested = [m.strip() for m in cfg.modes.split(",") if m.strip()]
+        unknown = [m for m in requested if m not in MODES]
+        if unknown:
+            raise ValueError(f"--modes has unknown {unknown}; valid: {MODES}")
+        run_modes = [m for m in MODES if m in requested]
+    else:
+        run_modes = list(MODES)
     rows = []
     for mode in run_modes:
         # load_problems applies the mode's factual hint; the elicit cell appends