tune(shaders): more dramatic Phase 3a lighting contrast

Original Phase 3a constants had SUN_DIR=(0.4,0.3,0.8) — heavily
vertical, so roofs and ground both landed near peak brightness and
only walls dropped. Combined with AMBIENT=0.4/DIFFUSE=0.6 the lit-vs-
shadow contrast was ~2.2x, which was real but hard to read through
textures. User feedback: "Ligtning looks the same I think."

Diagnosed with a temporary grayscale-lighting fragment output — walls
on different sides of the same building did show different brightness,
confirming the Phase 3a/3b pipeline is wired correctly end-to-end and
the issue was purely perceptual contrast.

Retuned: SUN_DIR=(0.5,0.4,0.6) (more oblique), AMBIENT=0.25, DIFFUSE=0.75.
Contrast ratio now ~3.3x. User-verified visually.

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

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

@@ -9,9 +9,13 @@ uniform sampler2D uDiffuse;
 // gives scenery and building faces enough differentiation to read as 3D instead
 // of looking like paper cutouts. Hardcoded for now; a later phase can route
 // light parameters through uniforms driven by the game's time-of-day.
-const vec3 SUN_DIR = normalize(vec3(0.4, 0.3, 0.8));
-const float AMBIENT = 0.4;
-const float DIFFUSE = 0.6;
+// Sun direction tuned after Phase 3a verification: (0.4,0.3,0.8) was too
+// vertical — roofs and ground both landed near peak brightness and only
+// walls dropped, so the contrast was hard to read through textures. More
+// oblique + lower ambient + higher diffuse = contrast ratio ~3.3x.
+const vec3 SUN_DIR = normalize(vec3(0.5, 0.4, 0.6));
+const float AMBIENT = 0.25;
+const float DIFFUSE = 0.75;
 
 void main() {
     vec4 sampled = texture(uDiffuse, vTex);