feat: implement recipient reward multiplier system (#880)

[jacobjacobb4242]

Issue #880 — Implement recipient reward multiplier system

Adds a new reward_multiplier_ext.rs module that lets an invoice creator attach
a per-recipient reward multiplier (in basis points) to control scaled payouts.

Core functionality

• set_reward_multiplier(creator, invoice_id, recipient, multiplier_bps) — creator sets a multiplier in [1, 20 000] bps (1× – 2×) for a recipient while the invoice is Pending.
• clear_reward_multiplier(creator, invoice_id, recipient) — resets to the default 1× (10 000 bps).
• get_reward_multiplier(invoice_id, recipient) — returns current multiplier or 10 000 if unset.
• get_all_multipliers(invoice_id) — enumerates every (recipient, multiplier_bps) pair on the invoice.
• effective_multiplier is exposed as pub(crate) for use by the release path.

Events

Topic	Data
(split, mul_set, invoice_id)	(recipient, multiplier_bps)
(split, mul_clr, invoice_id)	recipient

Storage

Local RewardMulKey enum in persistent storage:

• Multiplier(invoice_id, recipient) — the bps value.
• MultiplierRecipients(invoice_id) — enumeration list.

Tests

9 unit tests covering:

• Set and read a multiplier
• Default is 10 000
• Clear resets to default
• get_all_multipliers lists all entries
• Non-creator cannot set
• Zero multiplier rejected
• Multiplier > 20 000 rejected
• Unknown recipient rejected
• Multiplier readable after partial payment

Files changed

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

Closes #880

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