wip(L16, side branch): disk terrain cache design + headless Linux prototype

Exact lossless zlib tile records (keyed on world/season/recipe/plan/evaluator/options/tile),
pristine cache plus ordered edits, server never trusts client caches, corruption/stale rejection,
atomic writes, 2 GB cap. Loaded tiles match regenerated hashes (L1 view, 24 m crossing).
NOT for assembler/world yet: the shared StreamBenchmarkPipeline.h includes Linux-only headers
(unistd/fcntl/zlib), which would break the Windows viewer build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CSFYgsdW8ejZSyiMf4qRSX

Standalone/Source/StreamBenchmarkHeadless.cpp

@@ -35,6 +35,7 @@ namespace
         bool StreamBenchmark=false;
         bool StreamPipeline=false;
         bool StreamHash=false;
+        bool TileCachePrototype=false;
         bool ProfileColumnOverlap=false;
         bool UseSharedColumnCache=true;
         bool ForceGenerate=false;
@@ -119,6 +120,7 @@ namespace
                     "  --seed=N --recipe=surface-world|floating-islands\n"
                     "  --auto-surface-camera (place the 27-tile cube on this seed's surface)\n"
                     "  --mesh-coverage (report non-empty tiles and indexed triangles)\n"
+                    "  --tile-cache-prototype (opt-in exact disk-cache round trip; writes Saved/Linux/cache/l16-prototype)\n"
                     "  --screen-space-gate (measure display error and historical negative controls)\n"
                     "  --audit-uniform-block-proofs (fill and check every interval-certified block)\n"
                     "  --profile-lod-work (report per-LOD density sample counts)\n"
@@ -137,6 +139,15 @@ namespace
             else if(arg=="--stream-benchmark")options.StreamBenchmark=true;
             else if(arg=="--stream-pipeline")options.StreamPipeline=true;
             else if(arg=="--stream-hash")options.StreamHash=true;
+            else if(arg=="--tile-cache-prototype")
+            {
+                options.TileCachePrototype=true;
+                options.StreamPipeline=true;
+                options.StreamBenchmark=true;
+                options.WorldAssemblerPrototype=true;
+                options.ForceGenerate=true;
+                options.StreamHash=true;
+            }
             else if(arg=="--profile-column-overlap")options.ProfileColumnOverlap=true;
             else if(arg=="--force-generate")options.ForceGenerate=true;
             else if(arg=="--mesh-coverage")options.MeshCoverage=true;
@@ -377,5 +388,9 @@ int main(int argc,char** argv)
         cameraStepVoxels,options.AuditUniformBlockProofs,options.ProfileLodWork,
         options.RegionCrossing,options.AuditTileShapeReadiness,
         options.DiagnosticSlowShapeCompileMs,options.AuditBasePageEquivalence,
-        options.AuditRouteParity);
+        options.AuditRouteParity,
+        options.TileCachePrototype
+            ?std::filesystem::current_path()/"Saved/Linux/cache/l16-prototype"
+            :std::filesystem::path{},
+        options.DataPath);
 }