Wire `splitter.rs` into the crate and expose split-payment entrypoints

[Ibinola]

Description

The payment-splitting module exists on disk but is not declared from lib.rs and is not reachable through VeritixToken. This hides real code from CI and prevents the feature from being used.

Acceptance Criteria

• splitter.rs is declared from lib.rs
• VeritixToken exposes public create/distribute/get split methods
• The crate compiles with the splitter module included

Technical Scope

• Files likely affected: src/lib.rs, src/contract.rs, src/splitter.rs
• Modules/contracts involved: VeritixToken, splitter

Dependencies

None

Notes

Expect follow-up fixes once the module is actually compiled, because placeholder tests suggest it has never been exercised end-to-end.