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>

HANDOFF.md

@@ -5,7 +5,7 @@ everything they specified is shipped, and what still matters from them is record
 long history, `~/.claude/projects/.../memory/splatter-tennis-history.md` is auto-loaded; for the
 detail of any individual change, read the git log.
 
-Last updated 2026-07-31. **Start with "How to work on this", "OPEN — leftovers" and "LATER —
+Last updated 2026-07-31 (serve wind-up pass). **Start with "How to work on this", "OPEN — leftovers" and "LATER —
 shipping it"** (all three just above the CLOSED sections) — the first tells you how Oliver's feedback
 should be handled, the second is everything known to be outstanding in the game, the third is the
 demo/store/campaign plan, which is decided but entirely unbuilt. All four playtest rounds are closed
@@ -384,6 +384,18 @@ Clear the intro first: `document.getElementById('intro')?.remove()`.
    survives as a rim on a green court.
 12. **Two-row "style reference / redraw this" input does NOT work.** The model returns both rows and
    invents a background. Describe the target detail level in words instead.
+13. **`assets/_derive.py` is where a frame that must MATCH another frame is made.** Three of them
+   live there now: `p_bounce2` (a per-column shear of `p_bounce` — the free arm bends, nothing else
+   moves), `p_toss` (the drawn-in ball cut out, canvas height KEPT so drawChar keeps his scale) and
+   the runner's hip registration. "Same drawing, one small difference" is the one request the
+   generator cannot fill: it returns a different man and the swap reads as a flicker, which is the
+   defect you were fixing. The bounce/toss steps are idempotent; the runner step is `--runner` and
+   must run exactly once per generated set, because it mirrors and it pads.
+14. **A new pose needs a STANDING frame in the same sheet, as frame 0, thrown away after.** A sprite's
+   drawn size comes from its total height, so a sheet of nothing but arms-overhead poses has no
+   honest scale to anchor on and the man comes out short. `_regen.py`'s `ref` field does this, but
+   only when `ref_only` is unused — with `ref_only` set, list the throwaway as an extra sprite
+   (`p_pull0`) and delete the PNG afterwards.
 
 Known generator quirks, all still true:
 - It sometimes ignores the magenta (harmless — the key uses the corner colour).
@@ -397,6 +409,34 @@ Known generator quirks, all still true:
 - **Colour-match any regenerated frame** of an existing pair before shipping: fit a per-channel
   linear map onto the original's mean and sd, then verify with a mean-difference number.
 
+## Session log: the serve is the gesture (2026-07-31, after the art pass)
+
+Oliver's notes on the art pass: the zombies are good, the runner "flickers very hard", and the
+player has three problems. All four fixed, all four measured first.
+
+- **The pull IS the wind-up.** Four new frames (`p_pull1..4`, `assets/_regen_batches.json` batch
+  `player_pull`) — racket above the head, behind the head, behind the back, fully coiled — chosen in
+  `drawChar` off `pullFrac(G.aim.mag)`, the same 0..1 that sets the shot's power. Drag further, coil
+  further; release fires `p_smash`. Verified per stage with drawImage instrumentation:
+  mag 10/50/90/130 -> pull1/pull2/pull3/pull4.
+- **The bounce pair is now ONE drawing.** `p_bounce2` is `p_bounce` with the free arm sheared up 12
+  px (0.10 m) — everything else is pixel-identical, so the swap cannot flicker. The swap follows the
+  BALL now (`|sin| > 0.5`) instead of its rising phase, and `BOUNCE_IDLE` was re-measured off the
+  sprite (`apex` 0.55 -> 0.6, `dx` 0.5 -> 0.43) so the ball tops out under the raised palm instead of
+  passing beside it.
+- **`p_toss` had the tossed ball drawn into it.** Cut, without re-cropping the canvas: the game
+  scales him off `img.height`, so trimming the dead space would have shrunk him. Still 100x308.
+- **The runner.** Two measured defects, both in the PNGs, no code change. Its four frames were
+  bbox-cropped and drawn centred on the body's x, so hip centres sat -2.0/+7.1/-6.0/-0.1 px off
+  centre — a 13 px lateral jump at 12 frames a second, which is the flicker. And Oliver's own
+  diagnosis was right: all four frames leaned the same way and lifted the SAME leg, so the cycle
+  never used his other side. Frames 3 and 4 are mirrored (he is front-on, so a mirror costs nothing)
+  and all four are padded to one width with the hip on the centre line.
+
+Not done, and the honest reason: the runner's cadence is still time-driven (12 fps) rather than
+stepped off distance covered like every other body. It did not read as wrong once the frames stopped
+jumping, and it is one line if a playtest says otherwise.
+
 ## Session log: the art-resolution pass (2026-07-31)
 
 **What changed.** The whole character roster was redrawn at real pixel-art resolution, plus a batch