passages: memoized exact MC midpoints + wider conservative sweep bounds (L101, exact vs L100)

Off-grid census by caller (MC midpoints, skirts, far shells); all 63.0M sweep misses accounted
(1.9M exact-route fallbacks, 61.1M counted bound skips). Exact MC midpoint queries memoized by
float bits (Unreal enables it too); feature bounds expanded. L100/L101 whole-view hashes equal in
all four strata (v216 unchanged); 0 added/removed crossings; 15 capture pairs pixel-identical.
Near-ready @W12 L100 -> L101: FI 1.77 -> 1.65, TN 1.66 -> 1.54, SW 0.85 -> 0.84, VS 1.03 -> 1.03 s
(L98 2.28/2.23/1.02/1.11). Fast tier PASS; Linux UE host builds. Unreal arrival traces not obtained
(the runner wrongly applied the 6 GiB address-space cap to Unreal).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Source/StreamBenchmarkHeadless.cpp

@@ -52,7 +52,7 @@ namespace
         bool InstrumentTileDrain=false;
         bool ColdWorkerTimeline=false;
         bool Linux77Profile=false;
-        bool ExactMidpointDensityMemo=false;
+        bool ExactMidpointDensityMemo=true;
         bool ActiveLaneBatching=false;
         bool ParallelSignEdgeRepair=true;
         int ProofBlockCells=8;
@@ -1459,6 +1459,7 @@ namespace
                     "  --linux77-profile (Linux-only SIGPROF stack and route census)\n"
                     "  --profile-passage-composer (emit per-tile passage work counters)\n"
                     "  --exact-midpoint-density-memo (exact-input midpoint query memo A/B)\n"
+                    "  --no-exact-midpoint-density-memo (disable exact midpoint memo)\n"
                     "  --active-lane-batching --proof-block-cells=N (proof-masked density fill A/B)\n"
                     "  --parallel-sign-edge-repair --serial-sign-edge-repair\n"
                     "  --shape-helper-limit=8|12 (default 8; shape build A/B)\n"
@@ -1526,6 +1527,7 @@ namespace
             else if(arg=="--profile-passage-composer")
             {options.ProfilePassageComposer=true;options.InstrumentTileDrain=true;}
             else if(arg=="--exact-midpoint-density-memo")options.ExactMidpointDensityMemo=true;
+            else if(arg=="--no-exact-midpoint-density-memo")options.ExactMidpointDensityMemo=false;
             else if(arg=="--active-lane-batching")options.ActiveLaneBatching=true;
             else if(arg=="--parallel-sign-edge-repair")
             {options.ParallelSignEdgeRepair=true;}