One file owns the court: beats.js, PROJECT.md (né HANDOFF), and tests gate the build

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

CLAUDE.md

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+# Splatter Tennis — session rules
+
+1. **Read `PROJECT.md` first** — it is the only planning document: how Oliver's feedback is handled,
+   every invariant, every settled feel decision, what is open. Do not re-derive or contradict it.
+2. **`node test.js` must pass before every commit.** It is also a Dockerfile build stage, so a red
+   suite blocks the deploy — but run it locally first. New state-machine behavior gets a test.
+3. **Mode transitions live in `beats.js` only** (duel/horde/boss/changeover). One mode owns the
+   court; every `start*` refuses while another mode runs. A new mode starts there and refuses there.
+4. **Every push deploys** (Coolify → splatter-tennis.wuwi.one). Bump `main.js?v=N` in index.html on
+   every push or phones keep the old build. New top-level files must be added to the Dockerfile COPY.
+5. **Zero dependencies, no build step, no framework.** This is a settled decision — solve it with
+   what is already in the repo.