Add referral fee routing on buy so referrers earn a share of the protocol fee

[Chucks1093]

Summary

There is no on-chain incentive for users to bring new buyers to the platform. Adding an optional referrer: Option<Address> param to the buy function lets any wallet earn a portion of the protocol fee when someone they referred makes a purchase. The referrer's cut is taken from the protocol fee, so buyers pay nothing extra.

Scope

• Add referrer: Option<Address> parameter to the buy function
• When a referrer is provided, split the protocol fee: e.g. 80% to protocol treasury, 20% to referrer (configurable bps stored in protocol config)
• Add referral_fee_bps to protocol config, settable by admin, defaulting to 2000 (20% of protocol fee)
• Referrer cannot be the buyer themselves — revert with InvalidReferrer if equal
• Emit ReferralFeeEarned { creator_id, buyer, referrer, amount, ledger } event
• Add get_referral_fee_bps() -> u32 view function
• When referrer is None, full protocol fee goes to treasury as before

Acceptance Criteria

• Referrer receives correct share of protocol fee on buy
• Protocol treasury receives the remainder
• Total fee split equals 100% of the protocol fee (no leakage)
• Self-referral reverts with InvalidReferrer
• Omitting referrer preserves existing behavior
• ReferralFeeEarned event emitted with correct fields
• Admin can update referral_fee_bps within a valid range

ETA: 24 hours

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Coordinate on Telegram

[GBOYEE]

I'll take this. Adding optional referrer: Option<Address> param to buy function. Referrer earns a cut (e.g. 10%) of the protocol fee. Storing referrer→earnings mapping, adding claim_referral_earnings entrypoint. Tests: buy with referrer accrues fees, buy without referrer works, claim transfers earnings. Branch: feat/referral-fee

[Dominion116]

kindly assign this issue to me.