feat(render3d): an aspen quakes, and a tree is no longer bent like a bow

layer_flutter(l, v) gives a scatter layer a per-leaf tremble. The vertex stage offsets
each leaf by a phase taken from its own place on the card, so neighbouring leaves are
never in step, and writes the result out as a varying the fragment stage uses to flash
the leaf's pale underside as it turns - which is the part that reads, since a still
frame of a tremble is a still frame of nothing. One uniform, one varying, no extra pass.

And the sway itself was measured in METRES: hgt * hgt * 0.35 is right for a 40 cm
flower and puts ten metres of sideways into a 14 m trunk, so every tall tree in the
valley stood bent over like a fishing rod. It is a fraction of the model's own height
now - the tip moves a few per cent of the tree whatever the tree is, and the base does
not move at all.

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

packages/ludic.render3d/shaders/grass.mesh

@@ -38,6 +38,7 @@ out vec2 v_uv[];
 out float v_seed[];
 out vec2 v_rot[];
 out float v_hull[];
+out float v_quake[];   // grass does not quake; written so model.frag can read it
 
 const float CELL = 16.0;
 float hash1(vec2 p) { return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453123); }
@@ -153,6 +154,7 @@ void main() {
         v_seed[o] = seed;
         v_rot[o] = vec2(s, c_);
         v_hull[o] = hull;
+        v_quake[o] = 0.0;
       }
     }
     for (int q = 0; q < ROWS - 1; q++) {