Contracts: TimeArenaBuyRouter — DOUB, CL8Y, ETH, USDm entry paths

Context

DOUB-primary buy router with Kumbaya multi-asset entry. Parent epic: #238 (closed).

Relevant files

• Refactor contracts/src/TimeCurveBuyRouter.sol → TimeArenaBuyRouter.sol
• contracts/src/fixtures/AnvilKumbayaFixture.sol
• docs/integrations/kumbaya.md
• frontend/src/lib/timeCurveKumbayaSingleTx.ts

Acceptance criteria

• DOUB direct: buy(charmWad) pulls DOUB
• CL8Y: swap via CL8Y/DOUB Kumbaya pair → DOUB → buy
• ETH / USDm: multi-hop to CL8Y → DOUB
• Single-tx router: buyViaKumbaya targets DOUB output equal to gross DOUB need
• Balance-delta ingress preserved (#123 (closed))
• Chain-time swap deadlines (#83 (closed))

Verification checklist

• Forge fork/local: each pay asset path
• bash scripts/verify-timecurve-buy-router-anvil.sh replaced with arena equivalent