test: guard CLJS mixer boundary

[meekmachine]

Summary

• replace the stale mixer migration note with the current Polyester/Loom3 mixer ownership boundary
• add a CLJS mixer-boundary check that rejects frame-loop/runtime terms in CLJS source and verifies runtime host control effects stay exposed
• wire the boundary check into npm run test:cljs
• tighten the CLJS smoke test so animation schedule/update/seek/pause/resume/parameter operations are forwarded as host control effects
• update AGENTS.md with mixer-boundary guidance plus GitHub CLI REST/Markdown quoting fallbacks

Why

Issue #16 is mostly about preventing the wrong architecture from creeping back in. Polyester CLJS should plan and emit scheduled snippets/control effects; Loom3/Three should own clip construction and AnimationMixer.update(delta). This PR makes that boundary executable in CI and removes the stale doc that still described a reverted legacy baseline.

Validation

• git diff --check
• node scripts/check-cljs-mixer-boundary.mjs
• npm run typecheck
• npm test
• npm run test:cljs
• npm run build

Refs #16