perf(render3d): Vulkan draws find pipelines by integer and reuse descriptor sets

R3D_VK_PROF at the camp view (about 2950 draws a frame) showed the frame spent in bookkeeping:
19.5 ms a frame finding pipelines by string key and 16 ms building descriptor sets.

- Pipelines: a mesh carries an interned vertex-layout id (dropped only when an attribute's shape
  changes, not when an instance buffer is swapped), render state packs into an int and the pass
  formats into another; the program's last hit is tried first. The string path only builds.
- Samplers: each texture keeps the sampler for its parameters until they change.
- Descriptor sets: a program's last set is reused within the frame while its blocks and resolved
  textures are unchanged; a uniform write that repeats the value it already holds changes nothing.

Headless on the RTX 3070 Ti at 1920x1080: 21.7 -> 53.3 fps (pipelines 0.2 ms, sets 8.5 ms, inside
draws 10 ms a frame; OpenGL 114 fps). The camp frame is unchanged and validation-clean. OpenGL frames
byte-identical at the five viewpoints; 59 self-tests pass; VKRES OK.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/mesh.ludic

@@ -16,7 +16,8 @@ property Mesh {
   vbufs: words,              # every vertex buffer the mesh owns, freed with it
   n_vbufs: int = 0,
   cur_buf: int = 0,          # the buffer the next attribute reads (while building)
-  ibuf:  int = 0             # the instance buffer attached last
+  ibuf:  int = 0,            # the instance buffer attached last
+  vk_layout: int = 0         # the Vulkan backend's interned id for the recorded layout (0: not yet)
 }
 
 function mesh_draw(m: Mesh) -> void { gpu_draw_mesh(m) }