Release 0.5.0: leaf drop + start-assembled for Leaf Fall

Adds two opt-in capabilities to MakeabilityLabLogoLeafFall (the intro
behavior is unchanged when neither is used):

- dropLeaves(): drops every background grid triangle to the bottom of the
  screen into a pile (gravity-accurate fall, sideways drift, tumble to a
  random resting angle) while the logo stays fixed — its colored triangles
  (pinned grid cells), black M-shadow, translucent L, and outlines do not
  move. Tunable via groundStaggerMs / groundFallMinMs / groundFallMaxMs /
  groundPileSpread.
- startAssembled option: renders the grid + logo fully assembled from the
  first frame (skips the fall-in intro) so nothing animates until
  dropLeaves() is called. Intended for things like a custom 404 page.

Supporting change: buildIntroPieces now tags pinned grid pieces with
isLogoColor (and carries each piece's source tri) so the drop can keep the
colored logo fixed. Additive only — no change to colors, pinning, or motion;
ZoomFade and GridFade behavior is unaffected.

The Leaf Fall demo gains an 'F' key to trigger the drop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jonfroehlich