Split Ratio High-Precision Decimal Support

[Kingsman-99]

Description

The current integer basis-point system (10 000 = 100%) cannot represent ratios like one-third exactly, leading to rounding errors in three-way splits. The contract should support higher-precision ratios using a configurable denominator.

Technical Context

Add ratio_denominator: u64 to the Invoice struct in types.rs. Validate that all ratios are < ratio_denominator and sum to exactly ratio_denominator. Update the payout calculation in contracts/split/src/lib.rs to use amount * ratio / ratio_denominator with checked arithmetic. Default denominator is 10_000.

Acceptance Criteria

• Denominator of 3 with ratios [1, 1, 1] distributes exactly one-third each (within 1 stroop rounding)
• Remainder from rounding is awarded to the first recipient
• Ratios must be strictly less than the denominator value
• Existing basis-point invoices with denominator 10_000 continue to work
• Fuzz test in fuzz/ covers random denominators and ratio combinations
• All CI checks (cargo build --target wasm32-unknown-unknown, cargo test, cargo clippy) pass and the branch has no merge conflicts

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[temisan0x]

@temisan0x has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Good day team, please i’d like to take on this issue

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