Ablation: search.high 0.25 -> 0.15 (skip wildly-infeasible top probes)

Smoke on the real-output substrate measured feasible sampling_u = 0.0156 (TP2)
and 0.0742 (TP4, per-GPU 0.618 = 2.24x TP2). search.high=0.25 made the binary
search waste its two top probes (u=0.125/0.0625, always infeasible, admitting the
most long-output requests) on every trial. 0.15 keeps ~2x headroom over the TP4
boundary (0.0742) and trims ~15-20% of per-trial cost with identical feasibility
results; if a runtime-tuned config ever saturates 0.15 the harness search-high
saturation stop fires (informative, not silent).

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

configs/examples/dash0_qwen27b_ablation_harness_on.json

@@ -157,7 +157,7 @@
   },
   "search": {
     "low": 0.0,
-    "high": 0.25,
+    "high": 0.15,
     "tolerance": 0.001,
     "max_probes": 6,
     "sample_seed": 20260325,

configs/examples/dash0_qwen27b_ablation_naive_off.json

@@ -157,7 +157,7 @@
   },
   "search": {
     "low": 0.0,
-    "high": 0.25,
+    "high": 0.15,
     "tolerance": 0.001,
     "max_probes": 6,
     "sample_seed": 20260325,