feat: add creator brand loyalty program on-chain (#875)

[jacobjacobb4242]

Issue #875 — Add creator brand loyalty program on-chain

Adds a new brand_loyalty_ext.rs module implementing a fully on-chain
per-creator loyalty points program. Payers accumulate points proportional
to payments and can redeem them for token-unit discounts on future invoices.

Core functionality

Program lifecycle (creator):

• init_loyalty_program(creator, points_per_unit, max_discount_bps) — one-time setup; points_per_unit ≥ 1, max_discount_bps ∈ [1, 5 000].
• update_loyalty_program(creator, points_per_unit, max_discount_bps) — update params after creation.
• enable_loyalty_discount(creator, invoice_id) — opt a specific invoice into the loyalty discount program.

Points lifecycle:

• credit_loyalty_points(creator, payer, invoice_id, amount) — awards points using the formula amount × points_per_unit / POINTS_SCALE (POINTS_SCALE = 1 000 000). Callable by the creator after pay.
• redeem_loyalty_points(payer, creator, points_to_redeem) — burns points and returns a discount amount in token units, capped at max_discount_bps / 10 000 of the raw discount.

Read-only:

• get_loyalty_program(creator) → Option<LoyaltyProgram>
• get_loyalty_points(creator, payer) → i128
• is_loyalty_discount_enabled(invoice_id) → bool

Events

Topic	Data
(split, loy_init)	creator
(split, loy_upd)	creator
(split, loy_crd, invoice_id)	(payer, points)
(split, loy_rdm)	(payer, creator, points_redeemed, discount)

Storage

Local LoyaltyKey enum in persistent storage:

• Program(creator) — LoyaltyProgram config.
• Points(creator, payer) — i128 accumulated balance.
• DiscountEnabled(invoice_id) — bool flag.

New type

LoyaltyProgram { creator, points_per_unit, max_discount_bps, active }

Tests

8 unit tests:

• Init and get program
• Double init rejected
• Update program
• Credit and read points
• Redeem points (checks formula + cap)
• Redeem more than balance rejected
• Enable loyalty discount flag
• Default points are zero
• Excessive discount bps rejected

Files changed

File	Change
contracts/split/src/brand_loyalty_ext.rs	New
contracts/split/src/lib.rs	Added mod brand_loyalty_ext;

Closes #875

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