Add `# Examples` doc test to `InvoiceStatus::to_u8` and `from_u8`

[Kingsman-99]

Description

to_u8() and from_u8() in contracts/split/src/types.rs are utility methods used in compact storage encoding. Adding /// # Examples doc tests with a round-trip example makes the API self-documenting and allows cargo test --doc to verify the examples remain correct.

Acceptance Criteria

• Add a /// # Examples doc block to to_u8() showing InvoiceStatus::Released.to_u8() == 1
• Add a /// # Examples doc block to from_u8() showing InvoiceStatus::from_u8(1) == InvoiceStatus::Released
• cargo test --doc passes

Context

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