Extract invoice funding-percentage helper into calc.rs

[Kingsman-99]

Description

Computing funded * 10_000 / total (funding completion in basis points) is repeated in multiple places (_pay, auto_resolve, get_invoice_funding_progress, etc.). Extracting it into a single funding_bps(funded: i128, total: i128) -> u32 helper in calc.rs reduces duplication and makes the clamping logic testable in isolation.

Acceptance Criteria

• New pub fn funding_bps(funded: i128, total: i128) -> u32 added to calc.rs
• Returns 0 when total <= 0
• Clamps the result to [0, 10_000]
• All existing inline computations replaced with calls to this function
• At least three unit tests covering: funded < total, funded == total, funded > total
• cargo test passes

Context

• Target file: contracts/split/src/calc.rs

[Netty-kun]

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[Malik6828]

@Malik6828 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello, I can solve this issue.

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[grantfox-oss]

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@Malik6828's PR #721 was approved and merged by @Kingsman-99.

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