Contracts: Play CRED ledger, epoch CHARM, pro-rata CRED yield, claim on Last Buy reset

Context

Play CRED non-transferable token, epoch CHARM, and CRED claim flow. Parent epic: #238 (closed).

Relevant files

• New: contracts/src/PlayCred.sol
• contracts/src/TimeArena.sol — epoch CHARM mappings charmWad[epoch][user]
• New: contracts/src/interfaces/IPlayCred.sol

Acceptance criteria

• PlayCred — no transfer; MINTER_ROLE for TimeArena + admin grantor
• Optional CredGrantor contract admin can authorize
• buyWithCred(charmWad) burns 70 CRED from buyer; same min/max CHARM rules
• Each DOUB buy accrues 35 CRED to distribute: pro-rata by epoch CHARM weight
• claimCred(epoch) callable after Last Buy epoch ends; zeros charmWad[epoch][user]; transfers accrued CRED
• Buys still accrue CHARM into current Last Buy epoch (for CRED weighting)

Verification checklist

• Forge: two holders 1:2 CHARM → 35 CRED splits correctly
• Claim zeros epoch CHARM; cannot double-claim
• CRED buy respects charm min/max