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>

src/AcDream.App/Rendering/RenderBootstrap.cs

@@ -174,9 +174,16 @@ public static class RenderBootstrap
         var lightingUbo  = new SceneLightingUboBinding(gl);
 
         // --- Mesh shader (GameWindow ~1769-1771) ---
+        // Campaign V slice V6e: mesh_modern has needed common.glsl since V2 —
+        // ACDREAM_UBO_SET appears in a layout qualifier and ACDREAM_SAMPLE_ARRAY
+        // at the sample site, and without the preamble both are undeclared
+        // identifiers, so this program has failed to link on the Studio path
+        // since that slice. The world composition (WorldRenderComposition) has
+        // always passed true; this is the same pair loaded the same way.
         var meshShader = new Shader(gl,
             Path.Combine(shaderDir, "mesh_modern.vert"),
-            Path.Combine(shaderDir, "mesh_modern.frag"));
+            Path.Combine(shaderDir, "mesh_modern.frag"),
+            includeCommonPreamble: true);
 
         // --- TextureCache (GameWindow ~1774) ---
         var frameFlights = new GpuFrameFlightController(gl);