logarithmic march MEASURED and REFUTED: APS 6940 vs the published 2806 (2.5x worse), CHA 3115 vs the 2857 target, two sets 10414 vs <6026; the mechanism did work in the predicted direction (CHA 4516 -> 3115 = 1.45x) but the canonical set pays for it — the same trade that ended the four previous attempts, now measured for the transform too. Guarantee intact everywhere (over2x = 0)

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

sweep_full.py

@@ -159,10 +159,26 @@ def evaluate(spec):
     return res
 
 
+def _preflight_log_mode():
+    """Fail LOUDLY if the log mode is not actually available.
+
+    `evaluate` catches every exception and charges the full budget, so a stale
+    deployment (an old simroot/pure.py without `space`) looks exactly like a method
+    that solves nothing: on 2026-09-20 that produced a whole sweep of 39800 = 199 x
+    MAXFEV before the cause was found. One call up front turns that into an error."""
+    x, _n, ok = pure_root(lambda t: t - 1.0, 0.0, 3.0, 1e-10, 100,
+                          similarity="ratio", scale="derived3", guard="credit",
+                          space="log")
+    if not (ok and abs(x - 1.0) < 1e-8):
+        raise SystemExit("log mode preflight FAILED: the mode is present but does not solve "
+                         "a linear problem — refusing to run a sweep that would report zeros")
+
+
 def build_grid_log(smoke):
     """The logarithmic-space grid (PREREG 2026-09-20). Baselines are NOT recomputed:
     their numbers are already in sweep_full.json and re-running known expensive
     controls is waste. Only the derived scales are admissible in u-space."""
+    _preflight_log_mode()
     sims = ["ratio", "cbrt"] if smoke else sorted(PURE_SIMS)
     guards = ("credit", "project") if smoke else GUARD_MODES
     return [("pure", sim, sc, gu, "log")