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")
 }

packages/ludic.render3d/gpu_vk_draw.ludic

@@ -960,6 +960,14 @@ function gvk_clear_color(r: int, g: int, b: int, a: int) -> void {
   gvk_clear_rgba[0] = r;  gvk_clear_rgba[1] = g;  gvk_clear_rgba[2] = b;  gvk_clear_rgba[3] = a
 }
 function gvk_fb_colors(fb: int, n: int) -> void { if gvk_fb_ncolor != null and fb >= 0 and fb < 4096 { gvk_fb_ncolor[fb] = n } }
+# The framebuffer changes: a clear still waiting for this one's pass runs now, on this target,
+# rather than becoming the load op of whichever pass begins next.
+function gvk_rebind(fb: int) -> void {
+  if fb == gvk_fb_cur { return }
+  if not gvk_in_pass and gvk_clear_bits != 0 { gvk_pass_begin(gpu_fb, gpu_fb_at(gvk_fb_cur)) }
+  gvk_pass_end()
+  gvk_fb_cur = fb
+}
 function gvk_fb_forget(fb: int) -> void {
   if fb == gvk_fb_cur { gvk_pass_end() }
   gvk_fb_colors(fb, 1)

packages/ludic.render3d/scatter.ludic

@@ -341,6 +341,8 @@ var sc_printed: bool = false
 var sc_a2c: bool = true
 
 function scatter_init() -> void {
+  # R3D_DUMP_ATLAS: every impostor and card atlas the run bakes, to build/atlas_<n>_{color,alpha}.ppm
+  sc_debug_dump = Os.has_env("R3D_DUMP_ATLAS")
   sc_prog = r3d_program("model.vert", "model.frag", "")
   sc_prog_fol = r3d_program("model.vert", "model.frag", "#define FOLIAGE\n#define WIND\n#define ALPHA_TEST\n")
   sc_prog_fol_depth = r3d_program("model.vert", "depth.frag", "#define FOLIAGE\n#define WIND\n#define ALPHA_TEST\n")
@@ -413,6 +415,7 @@ function layer_add(l: Layer, x: int, y: int, z: int, scale: int, yaw: int, seed:
 
 # ---- impostors ---------------------------------------------------------------------
 var sc_bake_flower: bool = false
+var sc_dump_n: int = 0
 function impostor_bake(model: Model, tiles: int, tw: int, th: int) -> Impostor {
   let im = new Impostor
   im.tiles = tiles
@@ -475,7 +478,12 @@ function impostor_bake(model: Model, tiles: int, tw: int, th: int) -> Impostor {
   gpu_tex_param(GPU_TEX2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR)
   gpu_tex_mips(GPU_TEX2D)
   gpu_check("impostor bake")
-  if sc_debug_dump { tex_dump_alpha = true;  tex_dump(im.albedo, aw, th, "build/atlas_alpha.ppm");  tex_dump_alpha = false }
+  # numbered, so every bake of a run survives to be compared (a card layer per species bakes one)
+  if sc_debug_dump {
+    sc_dump_n += 1
+    tex_dump_alpha = true;  tex_dump(im.albedo, aw, th, `build/atlas_{sc_dump_n}_alpha.ppm`);  tex_dump_alpha = false
+    tex_dump(im.albedo, aw, th, `build/atlas_{sc_dump_n}_color.ppm`)
+  }
   return im
 }