feat(render): V6e — both particle shaders cross the dialect

Campaign V slice V6e, second of three. Billboard particles and mesh particles
are the last two pairs blocked on the texture-table shape; sky follows.

particle takes the same treatment mesh_modern took: the `flat uvec2` handle
varying becomes a `flat uint` slot and the fragment stage samples through
ACDREAM_SAMPLE_ARRAY. What is different here is the untextured particle. The
shader used to ask "is the handle I was given zero", which GL can answer because
its emulated table stores handles; Vulkan cannot, because set 2 is an opaque
descriptor array and reading an element nobody wrote is undefined rather than
zero. So the question moves to the index: the CPU writes ACDREAM_TEXTURE_NONE
for a particle with no texture instead of interning the null handle as a table
slot, and both dialects test the same value. GL renders identically — the same
particles take the same branch to the same procedural blob — and the handle
table simply stops carrying an entry that never named a texture. A test pins the
sentinel across all three declarations of it, because a silent disagreement here
would sample slot 0xFFFFFFFF instead of drawing the blob.

particle_mesh needed no restructuring, only names. Vulkan GLSL has no default
uniform block, so `uniform uint uTextureIndex;` is not unsupported but
unspellable, and the two values are per-pass — one texture and one layer for a
whole sub-batch — which is exactly what the shared push-constant block is for.
uTextureIndex becomes uTextureIndexA; uTextureLayer becomes uParamA, which was
the spare scalar and is a natural fit because the shader converted the layer to
float anyway. The widening moved from the shader to the CPU; layers are small
integers, so the sampled value is bit-identical.

Gates: Release build clean; App tests 4,058 passed / 3 skipped (baseline 4,057
plus the sentinel drift guard). Offline pixel gate against 95f8c25f: two
captures, 29 px and 21 px of 563,200 compared (3.73e-05 and 5.15e-05), with a
same-commit control between them of 13 px and this commit measuring 14 px
against its own parent. The scene draws no particles, so this gate is a tripwire
that the world path is undisturbed, not evidence about particles.

Particles remain user-gate debt — the same debt V2c and V4e already carry, to be
paid by casting a spell in a connected session.

Manifest: 6/9 pairs compile. Remaining: mesh (legacy, no consumer), sky (next
commit), terrain_modern.

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

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

@@ -53,25 +53,23 @@
     },
     {
       "name": "particle",
-      "vulkanReady": false,
+      "vulkanReady": true,
       "stages": [
         {
           "stage": "vert",
-          "sourceSha256": "6a6ebeaacba95e5e4e8a308ed7c4cd805b80f305650c1e9e03e2bdfc6c18f5e7",
-          "compiled": false,
-          "message": "particle.vert:85: error: \u0027assign\u0027 :  cannot convert from \u0027layout( location=6) in highp uint\u0027 to \u0027layout( location=2) flat out highp 2-component vector of uint\u0027"
+          "sourceSha256": "9629271f8997853a3c78a3cb1ec7af02a13518d68bdaa5ae1378997da7e5ab62",
+          "compiled": true
         },
         {
           "stage": "frag",
-          "sourceSha256": "3924ecbabf051349bc6a13e6cff370725a0832f8baf515decdb7e0394304006d",
-          "compiled": false,
-          "message": "particle.frag:62: error: \u0027sampler2DArray\u0027 : sampler-constructor requires the extension GL_ARB_bindless_texture enabled"
+          "sourceSha256": "bc08e4fb6f57da94d6c41d52aa81d73ab287c968214303a446c40a04d0495b40",
+          "compiled": true
         }
       ]
     },
     {
       "name": "particle_mesh",
-      "vulkanReady": false,
+      "vulkanReady": true,
       "stages": [
         {
           "stage": "vert",
@@ -80,9 +78,8 @@
         },
         {
           "stage": "frag",
-          "sourceSha256": "0da368243e967388990f4f4b90e2304044af6187de45f70499a3e4ece8dfd5a8",
-          "compiled": false,
-          "message": "particle_mesh.frag:64: error: \u0027uTextureIndex\u0027 : undeclared identifier"
+          "sourceSha256": "73c8db72aeb44e88054dd79fd722c7f683b27a8bc03368b5201f05adffd92728",
+          "compiled": true
         }
       ]
     },