perf(render3d): a depth prepass for the near tree foliage

Needle-card crowns are many cut-out quads deep and a discarding shader turns early depth
rejection off, so every card behind the front one ran the full lighting shader. The near
tree LODs now write depth first (depth.frag, the same alpha coverage test), terrain under
them is rejected before shading, and the lit pass draws them with no discard and an equal
depth test (invariant gl_Position). R3D_NOPREPASS=1 restores the old path.

Dense forest on a Windows PC: 61 -> 78 fps at 3840x2160, 116 -> 164 fps at 1920x1080.

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

packages/ludic.render3d/shaders/model.vert

@@ -40,6 +40,10 @@ float heightSmooth(sampler2D tex, vec2 uv) {
        + (texture(tex, vec2(o0.x, o1.y)).r * s0.x + texture(tex, vec2(o1.x, o1.y)).r * s1.x) * s1.y;
 }
 out vec3 v_wpos;
+// The foliage prepass (depth.frag) and the lit pass after it compile this same source
+// into two programs and compare depths for equality: the position must come out
+// bit-identical in both, which is what `invariant` promises.
+invariant gl_Position;
 out vec3 v_nrm;
 out vec2 v_uv;
 out float v_seed;