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>

changes/an-aspen-quakes.md

@@ -0,0 +1,15 @@
+bump: minor
+type: feat
+An aspen leaf hangs on a flattened stalk and turns in air a spruce never feels, and the
+kit had no way to say so. `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. The flash is the part
+that reads - a still frame of a tremble is a still frame of nothing. One uniform, one
+varying, no extra pass, and every other layer leaves it at zero.
+
+Also fixes the sway itself, which 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, so the tip moves a few per cent of the tree whatever the tree is and the base does
+not move at all.