Pure, headless easing curves of t in [0,1] for the organic-animation pass
(swap/fall/combine juice), placed alongside the existing ease_out_cubic /
ease_in_quad in board_anim.sx: ease_in_cubic, ease_in_out_cubic, ease_out_back
(bounded overshoot, settles to exactly 1), spring (damped wobble to exactly 1),
and squash_envelope (signed squash-&-stretch landing shape). The math module has
no exp/pow, so the decaying curves use a (1-t)^n polynomial envelope that hits 0
at t==1, pinning f(1) precisely.
Additive only: no render code calls the new curves yet. tests/easing.sx locks,
per curve, the endpoints, overshoot/undershoot bounds, and monotonicity-where-
required (booleans only, so the snapshot is platform-stable), structured so P16.2
can append illegal-swap bounce-back assertions. Test count 21 -> 22.
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