Document `SplitRule` enum variants with worked computation examples

[Kingsman-99]

Description

SplitRule in contracts/split/src/types.rs has three variants (Fixed, Percentage, Tiered) with brief labels but no examples showing how each is computed at release time. Adding worked examples to each variant's doc comment would make the enum usable without reading lib.rs.

Acceptance Criteria

• Add a /// # Example or inline example to each of Fixed, Percentage, and Tiered in contracts/split/src/types.rs
• Each example shows the computation: e.g. Percentage(3000) on funded = 10_000 yields 3_000
• cargo doc builds without warnings
• cargo test passes

Context

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