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
         }
       ]
     },

src/AcDream.App/Rendering/Shaders/terrain_modern.frag

@@ -5,14 +5,15 @@
 // Math identical to terrain.frag (Phase 3c per-cell maskBlend3 +
 // Phase G fog + lightning flash).
 //
-// Bindless texture handles are passed as uvec2 (low/high 32 bits) and
-// reconstructed into sampler2DArray at use sites via the GLSL
-// sampler-from-handle constructor. The alternative pattern —
-// `uniform sampler2DArray` set via glProgramUniformHandleARB — produces
-// GL_INVALID_OPERATION on at least one driver in practice (NVIDIA on
-// Windows). The uvec2 + constructor pattern is what N.5's mesh_modern
-// shader uses and is the documented "always works" form per the
-// ARB_bindless_texture spec.
+// Texture reads go through ACDREAM_SAMPLE_ARRAY (common.glsl) since slice
+// V6f-3, so this source compiles for both backends. Under GL that still expands
+// to the uvec2-handle + sampler2DArray-constructor pattern this shader has
+// always used — the documented "always works" form per the ARB_bindless_texture
+// spec, and the one that avoids the GL_INVALID_OPERATION the alternative
+// (`uniform sampler2DArray` set via glProgramUniformHandleARB) produces on at
+// least one driver in practice. Under Vulkan it indexes the set-2 descriptor
+// array instead. The extension requirement above is dropped for Vulkan by the
+// compiler's preamble, where it would be an error rather than a no-op.
 
 in  vec2  vBaseUV;
 in  vec3  vWorldNormal;
@@ -35,8 +36,23 @@ out vec4 fragColor;
 // push-constant plumbing yet, so these stay plain uniforms for now.
 uniform uint uTextureIndexA;
 uniform uint uTextureIndexB;
-#define uTerrain sampler2DArray(ACDREAM_TEXTURE_HANDLE(uTextureIndexA))
-#define uAlpha   sampler2DArray(ACDREAM_TEXTURE_HANDLE(uTextureIndexB))
+
+// Campaign V slice V6f-3: the two atlases are sampled through the
+// dialect-neutral table read instead of a GL sampler-from-handle constructor.
+// `sampler2DArray(handle)` is a GL_ARB_bindless_texture form with no Vulkan
+// equivalent — Vulkan's table is an opaque descriptor array in set 2, and there
+// is no handle to construct a sampler from. ACDREAM_SAMPLE_ARRAY asks the
+// question both dialects can answer ("sample table slot N at these
+// coordinates") and expands to the right thing on each.
+//
+// A SAMPLING macro, not a sampler-returning one, for the reason common.glsl
+// records: 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 that qualifier. The old `#define uTerrain
+// sampler2DArray(...)` was that shape textually, so keeping it would have
+// reintroduced the hazard at every use site.
+#define sampleTerrain(uvw) ACDREAM_SAMPLE_ARRAY(uTextureIndexA, uvw)
+#define sampleAlpha(uvw)   ACDREAM_SAMPLE_ARRAY(uTextureIndexB, uvw)
 
 // Campaign V slice V6f-2: the 36 per-layer tiling factors moved out of a loose
 // `uniform float uTexTiling[36]` and into the uniform buffer GpuBindingModel
@@ -97,23 +113,23 @@ vec4 combineOverlays(vec2 baseUV, vec4 pOverlay0, vec4 pOverlay1, vec4 pOverlay2
     vec4 t0 = vec4(0.0), t1 = vec4(0.0), t2 = vec4(0.0);
 
     if (h0 > 0.0) {
-        t0 = texture(uTerrain, vec3(baseUV * terrainTiling(pOverlay0.z), pOverlay0.z));
+        t0 = sampleTerrain(vec3(baseUV * terrainTiling(pOverlay0.z), pOverlay0.z));
         if (pOverlay0.w >= 0.0) {
-            vec4 a = texture(uAlpha, vec3(pOverlay0.xy, pOverlay0.w));
+            vec4 a = sampleAlpha(vec3(pOverlay0.xy, pOverlay0.w));
             t0.a = a.a;
         }
     }
     if (h1 > 0.0) {
-        t1 = texture(uTerrain, vec3(baseUV * terrainTiling(pOverlay1.z), pOverlay1.z));
+        t1 = sampleTerrain(vec3(baseUV * terrainTiling(pOverlay1.z), pOverlay1.z));
         if (pOverlay1.w >= 0.0) {
-            vec4 a = texture(uAlpha, vec3(pOverlay1.xy, pOverlay1.w));
+            vec4 a = sampleAlpha(vec3(pOverlay1.xy, pOverlay1.w));
             t1.a = a.a;
         }
     }
     if (h2 > 0.0) {
-        t2 = texture(uTerrain, vec3(baseUV * terrainTiling(pOverlay2.z), pOverlay2.z));
+        t2 = sampleTerrain(vec3(baseUV * terrainTiling(pOverlay2.z), pOverlay2.z));
         if (pOverlay2.w >= 0.0) {
-            vec4 a = texture(uAlpha, vec3(pOverlay2.xy, pOverlay2.w));
+            vec4 a = sampleAlpha(vec3(pOverlay2.xy, pOverlay2.w));
             t2.a = a.a;
         }
     }
@@ -125,12 +141,12 @@ vec4 combineRoad(vec2 baseUV, vec4 pRoad0, vec4 pRoad1) {
     float h1 = pRoad1.z < 0.0 ? 0.0 : 1.0;
     vec4 result = vec4(0.0);
     if (h0 > 0.0) {
-        result = texture(uTerrain, vec3(baseUV * terrainTiling(pRoad0.z), pRoad0.z));
+        result = sampleTerrain(vec3(baseUV * terrainTiling(pRoad0.z), pRoad0.z));
         if (pRoad0.w >= 0.0) {
-            vec4 a0 = texture(uAlpha, vec3(pRoad0.xy, pRoad0.w));
+            vec4 a0 = sampleAlpha(vec3(pRoad0.xy, pRoad0.w));
             result.a = 1.0 - a0.a;
             if (h1 > 0.0 && pRoad1.w >= 0.0) {
-                vec4 a1 = texture(uAlpha, vec3(pRoad1.xy, pRoad1.w));
+                vec4 a1 = sampleAlpha(vec3(pRoad1.xy, pRoad1.w));
                 result.a = 1.0 - (a0.a * a1.a);
             }
         }
@@ -152,7 +168,7 @@ vec3 applyFog(vec3 lit, vec3 worldPos) {
 void main() {
     vec4 baseColor = vec4(0.0);
     if (vBaseTexIdx >= 0.0) {
-        baseColor = texture(uTerrain, vec3(vBaseUV * terrainTiling(vBaseTexIdx), vBaseTexIdx));
+        baseColor = sampleTerrain(vec3(vBaseUV * terrainTiling(vBaseTexIdx), vBaseTexIdx));
     }
 
     vec4 overlays = vec4(0.0);