feat(render): V6e — move the world mesh's texture lookup to where Vulkan can express it

Campaign V slice V6e, first of three. mesh_modern is the shader every world
static, every piece of scenery and every EnvCell surface draws through, and it
was one of the four production pairs the SPIR-V toolchain still refused.

The blocker was a varying. Since V2 the vertex stage looked a batch's table slot
up in the binding=9 handle table and forwarded the resulting 64-bit
GL_ARB_bindless_texture handle to the fragment stage as a `flat uvec2`. That
works on GL because a bindless handle is just a number a shader may carry
anywhere. It cannot work on Vulkan at all: the equivalent object is a descriptor
in set 2, and a descriptor is not a value a stage can hand to another stage. So
what travels between the stages is now the SLOT — a `flat uint` — and the
fragment stage does the lookup at the point of sampling.

That relocation needs one shared idea, because the two backends disagree about
what the lookup IS. `ACDREAM_SAMPLE_ARRAY(slot, uvw)` asks the dialect-neutral
question — "sample table slot N" — and expands to
`texture(sampler2DArray(gTextureTable[slot]), uvw)` under GL and to
`texture(uTextures[nonuniformEXT(slot)], uvw)` under Vulkan. It is deliberately
a SAMPLING macro rather than a sampler-returning one: `nonuniformEXT` belongs on
the indexing expression itself, and binding the result to a local
`sampler2DArray` first is exactly where an implementation is free to drop it.
That is the same shape V6d already used for the retained UI's 2-D reads, and it
now covers the array reads the world path needs.

`ACDREAM_TEXTURE_NONE` lands alongside it, unused here and used by the next
commit. GL can ask "does this slot hold a texture" of the payload, because an
unregistered slot holds the null handle; Vulkan cannot, because set 2 is opaque
and reading an unwritten element of a partially-bound array is undefined rather
than zero. The sentinel moves that answer into the index, where both dialects
test it identically.

On GL nothing about the sampled result changes — the same slot resolves to the
same handle to the same texel. The SSBO read simply happens one stage later,
and `flat` keeps it one scalar load per primitive rather than per fragment.

Also: RenderBootstrap has been loading mesh_modern without common.glsl since V2,
which cannot have linked — `ACDREAM_UBO_SET` sits inside a layout qualifier
there. The UI Studio path is the only caller. One argument, same pair, same way
WorldRenderComposition has always loaded it.

Gates: Release build clean; App tests 4,057 passed / 3 skipped (baseline);
offline pixel gate against 95f8c25f differing fraction 3.37e-05 (~19 px of
563,200), inside the documented 15–23 px same-commit noise band and ~30x under
the 0.001 threshold. mesh_modern is the shader that gate covers most heavily,
so this is the strongest automated evidence any V6e commit gets.

Manifest: 4/9 pairs compile (debug_line, mesh_modern, ui_text, vk_probe).

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

tools/ShaderCompiler/VulkanGlslPreamble.cs

@@ -101,6 +101,17 @@ internal static class VulkanGlslPreamble
         // textures stay plain GL_TEXTURE_2D objects.
         text.AppendLine(
             "#define ACDREAM_SAMPLE_2D(idx, uv) texture(ACDREAM_TEXTURE(idx), vec3((uv), 0.0))");
+        // Slice V6e: the 2-D ARRAY read (world meshes, particles, terrain). GL
+        // reconstructs a sampler2DArray from the slot's bindless handle; Vulkan
+        // indexes set 2 directly. Sampling in one expression is what keeps the
+        // nonuniformEXT qualifier on the indexing operation itself.
+        text.AppendLine(
+            "#define ACDREAM_SAMPLE_ARRAY(idx, uvw) texture(ACDREAM_TEXTURE(idx), uvw)");
+        // Slice V6e: the reserved "no texture" slot. Under GL an empty slot can
+        // be recognised by the null handle it holds; a Vulkan descriptor array
+        // has nothing to compare, so the sentinel lives in the index and both
+        // dialects test it the same way. See common.glsl for the GL half.
+        text.AppendLine("#define ACDREAM_TEXTURE_NONE 0xFFFFFFFFu");
         text.AppendLine();
         text.AppendLine("// §3.4 push constants: one shared 96-byte block, so switching pipelines");
         text.AppendLine("// mid-pass invalidates neither descriptors nor constants.");