Harden routes and add browser playtest coverage - #14

Summary

• harden handcrafted and Daily Rex routes against edge landings, hazards, enemy/projectile timing, and checkpoint respawns
• stabilize the Molten Nest boss exit and Crystal Valley lava transition
• add a reusable Playwright browser harness plus regression coverage for gameplay systems

Local validation

• npm test — 308 tests passed
• npm run typecheck — passed
• npm run lint — passed
• npm run build — passed
• node scripts/playtest.mjs — all 5 handcrafted levels and Daily Rex passed
• node scripts/playtest.mjs dusk-stress — all checkpoint stress scenarios passed

GitHub Actions failures remain unrelated pre-existing issues, so the local validation above is the release gate.