perf(rendering): true DrawElementsInstanced — one draw call per (GfxObj × sub-mesh)

Replaces the per-entity glUniform uModel path with a shared instance VBO and
DrawElementsInstanced. All instance model matrices are uploaded to GPU once per
frame; the VAO's per-instance attribute pointers (locations 3–6, divisor=1) are
updated with a byte-offset re-point per group so a single VBO serves all groups
without requiring DrawElementsInstancedBaseInstance (not in Silk.NET 2.23).

Changes:
- InstancedMeshRenderer: add _instanceVbo, _instanceBuffer scratch; EnsureUploaded
  sets up mat4 instance attrs (locs 3–6) from the shared VBO; Draw builds the flat
  float[] of all instance matrices once then calls DrawElementsInstanced per sub-mesh.
  Drops the unused uint TerrainLayer attribute (loc 3 from vertex VBO) — mesh shaders
  never used it. Adds InstanceGroup helper to track per-group buffer offsets.
- mesh_instanced.frag: replace sampler2DArray+uTextureLayer with sampler2D uDiffuse,
  matching the existing TextureCache / individual-texture pipeline.
- mesh_instanced.vert+frag: track as committed files (were untracked).
- Shader.cs: add SetVec3 helper needed for uLightDirection uniform.
- GameWindow.cs: switch mesh shader load from mesh.vert/.frag to
  mesh_instanced.vert/.frag.

Visual output is identical: same entities, same textures, same lighting constants
(SUN_DIR=(0.5,0.4,0.6), AMBIENT=0.25, DIFFUSE=0.75 — moved from frag to vert).
Build: clean. Tests: 431/431 green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,31 @@
+#version 430 core
+
+in vec2 vTex;
+in vec3 vWorldNormal;
+in float vLightingFactor;
+
+out vec4 fragColor;
+
+// One 2D texture per draw call — same binding point as mesh.frag so the
+// C# side can use the same TextureCache without a texture-array pipeline.
+uniform sampler2D uDiffuse;
+
+// Translucency kind — matches TranslucencyKind C# enum (same as mesh.frag):
+//   0 = Opaque     — depth write+test, no blend; shader never discards
+//   1 = ClipMap    — alpha-key discard at 0.5 (doors, windows, vegetation)
+//   2 = AlphaBlend — GL blending handles compositing; do NOT discard
+//   3 = Additive   — GL additive blending; do NOT discard
+//   4 = InvAlpha   — GL inverted-alpha blending; do NOT discard
+uniform int uTranslucencyKind;
+
+void main() {
+    vec4 color = texture(uDiffuse, vTex);
+
+    // Alpha cutout only for clip-map surfaces (doors, windows, vegetation).
+    // Blended surface types must NOT discard here — that kills every
+    // semi-transparent pixel before the blend stage runs.
+    if (uTranslucencyKind == 1 && color.a < 0.5) discard;
+
+    // Apply pre-computed Lambert + ambient lighting factor from the vertex shader.
+    fragColor = vec4(color.rgb * vLightingFactor, color.a);
+}