wip(standalone): GPU step 8 partial -- GPU skirts, first-present hold; remote gap open

Luna's session ended on an API "Bad Request" before a final report; state is
unverified by me. In the log: GPU clipped-skirt emitter (count/scan/emit,
protected signs and critical air honoured), exposed-skirt audit passing at
the end; first-present stall reproduced on every launch (2.36-2.59 s, not
exe identity/idle/CPU run) and hidden by holding the CPU-ready frame until
GPU residency. Remote LOD3-4 band still differs from CPU; draw ranges and the
one inspected LOD3 tile match, cause not yet found (one-tile probe mid-fix).

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

Standalone/Core/include/VoxelForgeCore/VoxelForgeCore.h

@@ -255,6 +255,11 @@ namespace VoxelForgeCore
         Vec3i origin{};
         std::int32_t cells = 0, step = 0, sampleDim = 0;
         bool nearDetailEnabled = false;
+        // CPU-decided clipmap seam data. The GPU evaluates the ordinary far-field
+        // rock/air signs used to clip skirts; this mask only says which tile faces
+        // need a curtain, and the depth mirrors the CPU mesher in voxel units.
+        std::uint8_t visibleSkirtFaceMask = 0;
+        float skirtDepthVoxels = 0.0f;
         float strataLo = 0.0f, strataHi = 0.0f, strataBlend = 12.0f;
         std::shared_ptr<const FarFieldWorldInput> worldInput;
         // Sparse exception bitset: bit 1 forces route-protected air. Ordinary rock/air