feat(render): Campaign V slice V6f-3 - terrain's atlas reads cross the dialect

The last thing keeping terrain_modern out of SPIR-V was how it named its two
atlases:

    #define uTerrain sampler2DArray(ACDREAM_TEXTURE_HANDLE(uTextureIndexA))
    #define uAlpha   sampler2DArray(ACDREAM_TEXTURE_HANDLE(uTextureIndexB))

`sampler2DArray(handle)` is a GL_ARB_bindless_texture construction with no
Vulkan equivalent. Vulkan's table is an opaque descriptor array in set 2; there
is no handle, so there is nothing to construct a sampler from. The ten sample
sites now go through ACDREAM_SAMPLE_ARRAY, the dialect-neutral read V6e
introduced for mesh_modern, wrapped in two shader-local macros that keep the
call sites reading as "sample the terrain atlas" rather than "index the table".

They are SAMPLING macros, not sampler-returning ones, and that is not
cosmetic. Under Vulkan the expansion carries `nonuniformEXT` on the indexing
expression, and binding the result to a local sampler2DArray first is exactly
where an implementation may drop it. The old `#define uTerrain
sampler2DArray(...)` was textually that shape, so preserving it would have
reintroduced the hazard at every use site.

On GL nothing about the sampled result changes: the same slot resolves through
the same binding=9 table to the same handle to the same texel, and the macro
expands to the identical expression the shader wrote by hand.

With this, terrain_modern compiles for Vulkan and the manifest reads 8/9. The
remaining pair is `mesh`, which the campaign doc records as having no consumer
at all - so every production shader acdream actually draws with is now
Vulkan-expressible. That closes the obligation §5 recorded against V6e ("the one
production pair still not Vulkan-expressible after V6e is terrain_modern") and
it closes the shader half of V4d's parked content.

What this does NOT do is give the Vulkan backend a world to draw. That is
reported separately with the rest of slice V6f; the shaders were the part that
could be finished, gated and landed on GL today.

Gates. Release build clean. App tests 4,073 passed / 3 skipped over four
consecutive runs. A fifth run failed only
CurrentRenderSceneOracleTests.SurfaceOverrideFingerprint_DictionaryHotPathAllocatesNothing,
an allocation-counting test over a CPU dictionary path that touches nothing in
this diff; it passes in isolation and passed in every other full run. That is
the known #250 flake class on an otherwise unchanged tree. Offline pixel gate
against fac09407: 12 differing pixels of 563,200 compared (fraction 2.13e-05) -
below even the floor of the documented 15-23 pixel band. Cumulatively, across
all three V6f commits against 7faaaa34: 22 pixels (3.91e-05, maximumChannelDelta
52), which is the same number the first commit measured on its own. Three
changes to the shader terrain draws with, and the drift has not accumulated.

No divergence-register row: the sampled result is unchanged on GL and no
retail-facing behaviour moves.

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

src/AcDream.App/Rendering/Shaders/spv/shaders.manifest.json

@@ -101,7 +101,7 @@
     },
     {
       "name": "terrain_modern",
-      "vulkanReady": false,
+      "vulkanReady": true,
       "stages": [
         {
           "stage": "vert",
@@ -110,9 +110,8 @@
         },
         {
           "stage": "frag",
-          "sourceSha256": "dc4d38b5eb356629c83681fbc6a1d5d867792e99a8c4e659031f390333edb7b8",
-          "compiled": false,
-          "message": "terrain_modern.frag:150: error: \u0027sampler2DArray\u0027 : sampler-constructor requires the extension GL_ARB_bindless_texture enabled"
+          "sourceSha256": "21f41dcca4d4973ad9ee148c5433e0dae015489414c164c51bde958963f03598",
+          "compiled": true
         }
       ]
     },