render3d: a fog wall nearer than a layer's cards lets its impostor atlases go, and a fog that opens bakes them again

Inside the wall every instance is a mesh whose LOD2 casts its shadow, so a card is neither drawn nor
cast there: fog_impostors.ludic frees a layer's atlases while wall + margin < its near (trees 420-480
m, rocks 320 m) and paints them again from the model when it opens past it (impostor_paint, split out
of the bake). Impostor atlases get memory of their own, so a release goes back to the driver instead
of leaving a hole in a shared block. R3D_FOG_AT=<frame> with R3D_FOG_TO=<m> changes the wall mid-run.

At the overlook: 2102 MB off, 1824 MB at 30 m (249 -> 67 MB of atlases; the rest is the ground cover's
card atlases, drawn inside the wall); opening re-bakes 16 layers in 42-46 ms, back to 2102 MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/gpu_vk.ludic

@@ -429,7 +429,9 @@ function gvk_mem_new(render3d_st: mut Render3dState, req: bytes, want: int, imag
   # aligned: carved at the bare size, every small buffer left a sliver before the next one that no
   # later one could use - 3,718 of them among 3,968 buffers, and the free list past its bound
   let span = (size + align - 1) / align * align
-  if size <= gvk_own_over(render3d_st) {
+  # an impostor atlas has memory of its own: a fog wall lets it go (fog_impostors.ludic), and inside a
+  # shared block its room went back to nobody while the block's other tenants stayed
+  if size <= gvk_own_over(render3d_st) and render3d_st.gvk_tag != VKM_IMPOSTOR {
     var kind = t * 2
     if image { kind += 1 }
     var i = 0