feat(app): directional lighting on terrain and static meshes (Phase 3a)

Adds a hardcoded sun direction + ambient + Lambert diffuse to both
terrain.frag and mesh.frag. Both vertex shaders now forward a world-
space normal (computed as mat3(uModel) * aNormal) for the fragment
shader to dot against the sun vector.

Lighting model:
  final_rgb = texture_rgb * (AMBIENT + DIFFUSE * max(0, dot(N, SUN)))
where AMBIENT=0.4, DIFFUSE=0.6, SUN=normalize(0.4,0.3,0.8).

Building walls facing the sun light up, walls in shadow dim to ~40%.
Scenery (trees, bushes, rocks) with real per-vertex normals from SWVertex
shades naturally. Terrain currently uses flat UnitZ normals so every
terrain fragment gets the same contribution — terrain will look a bit
washed out compared to real AC until a Phase 3b pass computes per-vertex
landblock normals from the heightmap.

Non-uniform scale (from scenery's random scale baked into MeshRef
PartTransform) would technically require the inverse-transpose for
correct normals, but scenery uses uniform scale so mat3(uModel) is
good enough. Flagging as a known Phase 3+ concern if nonuniform scale
ever shows up.

Build clean, runtime clean: 1133 entities hydrated, no shader compile
errors, process runs through startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Shaders/mesh.vert

@@ -8,8 +8,14 @@ uniform mat4 uView;
 uniform mat4 uProjection;
 
 out vec2 vTex;
+out vec3 vWorldNormal;
 
 void main() {
     vTex = aTex;
+    // Transform the mesh normal into world space. For uniform-scale transforms
+    // (the common case), the upper-left 3x3 of uModel is correct. Non-uniform
+    // scale would require the inverse transpose; we accept that as a Phase 3+
+    // concern.
+    vWorldNormal = normalize(mat3(uModel) * aNormal);
     gl_Position = uProjection * uView * uModel * vec4(aPos, 1.0);
 }