wip(assembler): M3 round 3 -- separated islands + Unreal island route (owner build 18:46)

Islands read as separate (91.89% open sky at mid-height), underside cavity closed,
rim teeth were capture shading. Island manifest import + BuildHost proof passed on the
pre-cache tree; M1 SurfaceWorld and M2 V1/V2 gates passed.

Known open: final XY-column cache compiled in the owner build but is unverified;
island LOD0 p95 ~155 ms misses the 133 ms sprint deadline (legacy ~43 ms); the island
manifest only fits the BuildHost fixture layout (owner's world: Core init fails,
empty world) -> round 4 builds plans inside Unreal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Source/OwnerWorldLoader.h

@@ -299,6 +299,17 @@ namespace StandaloneOwnerLoader
             Error = "export voxel_size_cm does not match the standalone VOXEL_SIZE";
             return false;
         }
+        const VFJson::Value& OwnerVoxelSize = Root.Get("owner_voxel_size_cm");
+        const VFJson::Value& CoreVoxelSize = Root.Get("core_voxel_size_cm");
+        const VFJson::Value& OwnerToCoreScale = Root.Get("owner_to_core_length_scale");
+        if (!std::holds_alternative<std::nullptr_t>(OwnerVoxelSize.Data)
+            && (!OwnerVoxelSize.IsNumber() || OwnerVoxelSize.Number() != 12.5
+                || !CoreVoxelSize.IsNumber() || CoreVoxelSize.Number() != 25.0
+                || !OwnerToCoreScale.IsNumber() || OwnerToCoreScale.Number() != 0.5))
+        {
+            Error = "owner-to-Core import scale must be 12.5 cm to 25 cm at a 0.5 length ratio";
+            return false;
+        }
         Out.Seed = static_cast<int32>(Root.Get("seed").Integer());
         Out.ClipRadius = static_cast<int32>(Root.Get("clip_radius").Integer(6));
         Out.MaxClipLevel = static_cast<int32>(Root.Get("max_clip_level").Integer(8));