render_fall now offsets each COLUMN's drop START by a small bounded delay
(fall_stagger_t) so a refilled/collapsed row pours in as a left-to-right cascade
instead of every gem snapping down in one flat lockstep row. Column col waits
FALL_STAGGER_MAX (0.30) * col/7 of the fall window, then falls over the remaining
1 - 0.30, with that local progress fed through ease_in_cubic so each column still
accelerates under gravity within its own window.
Bounded by construction: the last column lands EXACTLY at t=1 and every earlier
column strictly before it, so no gem is ever left mid-air at the segment end — the
seam to the next round / settled board stays invisible and move.final is untouched.
FALL_ANIM_DUR (0.22s) and the timeline helpers (phase/total/cascade_rounds_started)
are unchanged, so the per-round cascade-cue timing snapshots don't churn and live
per-round audio is unaffected. Render-only — no board.sx model change.
tests/easing.sx pins fall_stagger_t: f(0)=0, f(1)=1 across all columns (no gem
unlanded), per-column monotonicity, and the mid-fall cascade ordering (each later
column strictly behind the one before). tests/anim_plan.sx (final==model,
contiguity) stays green.
Golden goldens/p17_stagger.png: M3TE_FX=11 (depth-5 cascade, seed 1337) pinned at
M3TE_ANIM_TIME=1.91 — round 4 refills columns 2-7 by one cell each, so the top row
reads as a left-to-right staircase (vs the pre-stagger flat row in p17_fall.png).
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