Contracts: Referrals earn Play CRED (5% + 5%) instead of CHARM weight

Context

Adapt referrals to mint Play CRED instead of CHARM weight bonuses. Parent epic: #238 (closed).

Relevant files

• contracts/src/ReferralRegistry.sol
• contracts/src/TimeArena.sol
• docs/product/referrals.md
• frontend/src/pages/referrals/

Acceptance criteria

• Existing codeHash → owner mappings preserved in redeploy or merkle/import script documented
• On referred buy: mint 5% of the 35 CRED mint to referrer and 5% to buyer
• Self-referral still reverts
• Indexer ReferralApplied schema updated

Verification checklist

• Forge: referred buy mints CRED not CHARM
• /referrals page shows CRED earnings not CHARM