feat(render): Campaign V slice V2b - terrain texture-index migration

Continues the V2a mesh-path conversion onto TerrainModernRenderer: its two
per-pass bindless texture handles (the terrain atlas and the alpha-mask
atlas) now travel as table indices instead of raw 64-bit
ARB_bindless_texture handles, with zero pixel change.

Terrain differs structurally from the mesh path: it has no per-batch SSBO at
all, just two handles set once per draw as plain uniforms
(terrain_modern.frag's uTerrainHandle/uAlphaHandle, reconstructed via the
sampler2DArray(handle) macros uTerrain/uAlpha). So instead of a BatchData
struct field, the two uniforms became uTextureIndexA/uTextureIndexB - named
to match the pinned GpuPushConstants.TextureIndexA/B fields (campaign doc
section 3.4) so V4d's eventual move to push constants is a rename, not a
redesign. There is no push-constant plumbing yet, so these stay plain
uniforms for now, set via ProgramUniform1 instead of ProgramUniform2.

TerrainModernRenderer owns its own GlBindlessHandleTable and binding=9 SSBO
(the same GL-only handle-table emulation V2a introduced), independent of
WbDrawDispatcher's and EnvCellRenderer's - nothing requires index agreement
between renderers, and terrain only ever registers two handles per draw
(the atlas's terrain/alpha textures), so its table is dirty only once, on
first draw. Unlike WbDrawDispatcher/EnvCellRenderer, TerrainModernRenderer
already eagerly creates its other GL resources in the constructor with a
ResourceCleanupGroup rollback, so the texture-table SSBO is created there
too rather than lazily.

TerrainAtlas needed no change: GetBindlessHandles() keeps returning the raw
(ulong terrain, ulong alpha) pair unchanged - the table lookup is entirely a
TerrainModernRenderer-side concern, added at the one draw-call site that
already converts those handles into shader state.

Shader-side: terrain_modern.frag's uTerrain/uAlpha macros now expand through
common.glsl's ACDREAM_TEXTURE_HANDLE(idx) lookup; both terrain_modern.vert
and .frag opted into the common.glsl preamble (Shader's
includeCommonPreamble, introduced at V2a) so their SceneLighting UBO
declarations could also pick up the ACDREAM_UBO_SET scaffolding macro -
terrain_modern.vert doesn't touch the texture table itself, but sharing the
same preamble across both stages of a technique is simpler to reason about
than deciding per-stage.

Gate: dotnet build -c Release green, dotnet test tests/AcDream.App.Tests
-c Release green (3843 passed / 3 skipped, matching V2a), and
tools/run-offline-pixel-gate.ps1 passed against the V2a commit's build with
a 2.49e-05 differing-pixel fraction - within the documented ~33x same-commit
noise margin. No divergence-register row: this introduces no retail
behavior deviation.

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

src/AcDream.App/Composition/WorldRenderComposition.cs

@@ -209,7 +209,8 @@ internal sealed class RetailWorldRenderCompositionFactory
         new(
             gl,
             Path.Combine(shadersDirectory, "terrain_modern.vert"),
-            Path.Combine(shadersDirectory, "terrain_modern.frag"));
+            Path.Combine(shadersDirectory, "terrain_modern.frag"),
+            includeCommonPreamble: true);
 
     public SceneLightingUboBinding CreateSceneLighting(GL gl) => new(gl);