Contracts: DOUB prize split (70%) + AdminSellVault (30%)

Context

Per-buy DOUB prize routing and admin sell vault. Parent epic: #238 (closed).

Relevant files

• New: contracts/src/PodiumVaults.sol or modules in TimeArena
• New: contracts/src/AdminSellVault.sol
• docs/integrations/kumbaya.md

Acceptance criteria

• On each buy: 10% DOUB → each of 4 podium active pools; 7.5% → each of 4 seed pools; 30% → AdminSellVault
• Integer math: remainder to admin vault or largest pool (documented)
• AdminSellVault.sellDoubToUsdm(minOut) — only admin; USDM to admin account
• Events for indexer: PodiumFunded, SeedFunded, AdminVaultFunded

Verification checklist

• Forge: 1000 DOUB buy → vault balances 100/75/100/75/... + 300 admin
• Admin sell mocked with AnvilKumbayaFixture