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