PLAN.md: phased zombie-mode expansion plan for follow-up implementation

Five phases, each independently shippable with verify recipes: tighter
shooting loop, multi-kill umpire fanfare, procedural gore piles, bomb
balls delivered by ball boys, horde waves with machine-gun racket.
Racket zombies deferred to a design checkpoint (Phase 6, not first cut).

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

HANDOFF.md

@@ -38,7 +38,12 @@ Two runtime-only import cycles (documented in headers): input↔gameplay, zombie
   Ball boys mop gore they pass. HUD shows THEM instead of CPU.
 - Balance knobs in TUNE (T panel): zSpeed, zSpawnEvery, zMax, zSplatQ, zPushback.
 
-## ACTIVE TASK — all four items from Oliver's feedback round are DONE and VERIFIED
+## NEXT UP — PLAN.md
+The zombie-mode expansion (tighter shot loop, multi-kill fanfare, gore piles, bomb balls,
+horde waves + machine-gun racket, racket zombies later) is fully planned in PLAN.md —
+phased, with per-phase verify recipes. Implement it phase by phase, push after each.
+
+## PREVIOUS TASK — all four items from Oliver's feedback round are DONE and VERIFIED
 (serve cadence · flick-aim anywhere incl. over the net · zombie speed ramp · eaten):
 
 1. **Serve cadence — verified.** Dead splatter ball → 15 frames (0.25s) to next pretoss;