Add module-level doc-comment to math.rs explaining the normalization flow

[Kingsman-99]

Description

math.rs has a module-level comment but it does not describe the full normalization flow: when normalization is applied, what happens to fractional stroop remainders, and how callers should use normalize_amount paired with denormalize_amount. This context is critical for contributors extending the payment logic.

Acceptance Criteria

• Module-level //! comment added or expanded with a clear explanation
• Comment covers: why 7-decimal canonical scale was chosen, what truncation occurs for tokens with fewer than 7 decimals, and how callers should pair normalize/denormalize
• CANONICAL_DECIMALS constant has a doc-comment explaining its value
• cargo doc --no-deps completes without warnings

Context

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

[Netty-kun]

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

Hi maintainer, I have read and understood the issue requirements and checked the necessary documentation. I’m confident I can work on this task effectively and deliver a proper solution.
Happy to discuss the approach first if that's useful before I start.

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[gabrielujelistic-collab]

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

i would love to work on this

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