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/skin.vert

@@ -21,6 +21,7 @@ out vec2 v_uv;
 out float v_seed;
 out vec2 v_rot;
 out float v_hull;
+out float v_quake;   // only aspens quake; written so model.frag can read it
 void main() {
   mat4 m = u_model;
   if (u_skinned > 0.5) {
@@ -43,6 +44,7 @@ void main() {
   v_seed = 0.0701;
   v_rot = vec2(0.0, 1.0);
   v_hull = 1.0;
+  v_quake = 0.0;
 #ifdef SHADOW_PASS
   gl_Position = u_light_vp * w;
 #else