wip(assembler): M2 round 3 -- opt-in hybrid sharp mesher (WorldAssemblerHybridV1) in the Unreal Core route

Default MarchingCubesV1 unchanged (W1/W8/W16 = round-6 hashes). Hybrid DC only in LOD0 interior cells
with tagged features; tile-border cells and LOD1+ stay MC. BuildHost: fixtures repeat exactly, cooked
collision bounds = render bounds, same-LOD borders pass, sampled SurfaceWorld cliff bit-identical to MC.
Open: tagged tiles cost 5.7-6.7x MC; corner-top field contact error 20 cm; 5 MC doorway LOD cases.

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

Standalone/DESIGN_WORLD_ASSEMBLER.md

@@ -306,6 +306,21 @@ Impacts to carry into the mesher proof:
   corner invites snagging and makes capsule clearance disagree with what the player sees. Mesh atoms
   that players can stand on need an explicit matching proxy.
 
+### Round-3 implementation boundary
+
+The first Unreal recipe keeps this feature-aware DC path deliberately narrow: only tagged type-2 plan
+boxes participate, only at LOD0, and cells touching a tile border use Marching Cubes. LOD1 and coarser
+tiles also use Marching Cubes, so each 2:1 boundary is MC-to-MC and the existing skirt remains the
+visual repair. The standalone 625-case transition matrix still has five MC doorway topology failures;
+this compromise reduces the failure surface but does not close those cases or establish a transition
+template.
+
+At 25 cm, the worst phase bevel measured for MC's analytic 90-degree corner is about 17.68 cm. That is
+0.35 of a 50 cm LOD1 sample, so a coarse tile cannot preserve the same edge detail. The bevel can
+therefore round off and pop when the player crosses the LOD threshold. BuildHost uses null RHI, so no
+projected or in-game pop measurement was made; the owner needs to judge this at the target camera and
+movement speed.
+
 ## B. Bounds: hybrid certificates, not one universal Lipschitz number
 
 I agree with Lipschitz bounds as the cheap, generic first certificate. I do **not** recommend them as