Wind the serve up with the pull, and stop the frames flickering

Four new frames (p_pull1..4) turn the slingshot drag into the serve's wind-up:
racket above the head, behind the head, behind the back, fully coiled, picked off
pullFrac — the same 0..1 that sets the shot's power — so the gesture and the man
are one movement and the release snaps to the smash.

The two bounce frames are one drawing now. p_bounce2 is p_bounce with the free
arm sheared up 12px and nothing else touched, so the swap cannot flicker, and it
follows the ball instead of its rising phase; BOUNCE_IDLE was re-measured off the
sprite so the ball tops out under the palm. p_toss had the tossed ball drawn into
it — cut, canvas height kept, because the game scales him off img.height.

The runner: hip centres sat -2.0/+7.1/-6.0/-0.1 px off the sprite centre, a 13px
lateral jump at 12 frames a second, and Oliver's diagnosis was right that all four
frames leaned the same way and lifted the same leg. Frames 3 and 4 mirrored, all
four padded to one width with the hip on the centre line.

assets/_derive.py holds the three derivations, since "the same drawing with one
small difference" is the one thing the generator cannot draw.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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