Cut the serve down to three poses

Oliver, after playing the four-stage version on the phone: idle, then the racket
dropping on the pull, then the drive on release — "that's all, forget all the
others". At thumb speed the middle stages are never read, so they are deleted
rather than kept around: p_pull is the wind-up, p_smash is the drive, and both
come out of the photo-referenced sheet, so the whole serve is one drawing style.

The dead batches and the arm_left guard go with them. assets/_pose_ref_serve.png
is kept as the recipe: his photo sequence over our own sprites, which is what
turned a fifteen-generation prose argument into one generation.

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

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