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/terrain.frag

@@ -1,10 +1,25 @@
 #version 430 core
 in vec2 vTex;
 in flat uint vLayer;
+in vec3 vWorldNormal;
 out vec4 fragColor;
 
 uniform sampler2DArray uAtlas;
 
+// Phase 3a: shared lighting model with mesh.frag. Terrain normals are currently
+// flat UnitZ (Phase 2b set this when building LandblockMesh vertices) so every
+// terrain fragment gets the same ndotl contribution — terrain will look a bit
+// flatter than hill-shaded terrain, which is a Phase 3b concern (compute
+// per-vertex normals from the 9x9 heightmap to get relief lighting).
+const vec3 SUN_DIR = normalize(vec3(0.4, 0.3, 0.8));
+const float AMBIENT = 0.4;
+const float DIFFUSE = 0.6;
+
 void main() {
-    fragColor = texture(uAtlas, vec3(vTex, float(vLayer)));
+    vec4 sampled = texture(uAtlas, vec3(vTex, float(vLayer)));
+
+    vec3 N = normalize(vWorldNormal);
+    float ndotl = max(dot(N, SUN_DIR), 0.0);
+    float lighting = AMBIENT + DIFFUSE * ndotl;
+    fragColor = vec4(sampled.rgb * lighting, sampled.a);
 }