fix(shaders): varyings meet by name on Vulkan, so the meadow's flowers draw

OpenGL links a vertex output to a fragment input by name; SPIR-V links them by location, and
glslang's --auto-map-locations numbered each stage in its own declaration order. The foliage
prepass's depth.frag declares v_wpos then v_uv where model.vert writes v_wpos, v_nrm, v_uv, so
the prepass read a normal as its texture coordinate, its alpha test cut every flower head and
leaf, and the lit pass (depth EQUAL) drew nothing over them. `ludic-dev shaders` now gives both
stages explicit locations: the vertex stage's out order numbers them and the fragment stage looks
each in up by name. All 45 variants checked: every fragment input sits on its vertex output.

Also:
- A clear still waiting for its pass when the framebuffer changes now runs on that framebuffer,
  instead of becoming the load op of whichever pass began next.
- R3D_DUMP_ATLAS writes every impostor and card atlas a run bakes (build/atlas_<n>_*.ppm); the
  40 baked on Vulkan match OpenGL's.

The PC's Vulkan frame now shows the flowers as OpenGL does, validation-clean. ludic-dev test 140
passed; OpenGL frames byte-identical at the five viewpoints; 59 self-tests pass.

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

packages/ludic.render3d/gpu.ludic

@@ -738,7 +738,7 @@ function gpu_debug_check(tag: string) -> void { if gpu_kind == GPU_VK { return }
 
 function gpu_fb_new() -> int { if gpu_kind == GPU_VK { gvk_fb_counter += 1;  return gvk_fb_counter };  return gl_framebuffer() }
 function gpu_fb_bind(fb: int) -> void {
-  if gpu_kind == GPU_VK { if fb != gvk_fb_cur { gvk_pass_end() };  gvk_fb_cur = fb } else { gl_bind_framebuffer(GL_FRAMEBUFFER, fb) }
+  if gpu_kind == GPU_VK { gvk_rebind(fb) } else { gl_bind_framebuffer(GL_FRAMEBUFFER, fb) }
   gpu_fb_cur = fb
   gpu_glcheck_after("fb bind")
 }