Convert `InvoiceHot` block comment to rustdoc `///` comment with `# Design` section

[Kingsman-99]

Description

InvoiceHot in contracts/split/src/types.rs has an inline code comment explaining the design rationale but it is inside a comment block rather than a rustdoc /// comment. Converting it to a /// doc comment with a # Design section ensures it appears in generated documentation and is visible in IDE hover tooltips.

Acceptance Criteria

• Convert the existing block comment above InvoiceHot to a /// rustdoc comment
• Add a /// # Design section explaining instance vs persistent storage and the TTL trade-off
• cargo doc builds without warnings
• cargo test passes

Context

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

[lonerthefirst3-sudo]

@lonerthefirst3-sudo 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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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@lonerthefirst3-sudo's PR #660 was approved and merged by @Kingsman-99.

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[drips-wave]

This issue has been completed by @lonerthefirst3-sudo as part of the Stellar Wave Program's 8th Wave 🥳

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