Add module-level doc comment to `events.rs` explaining event naming convention

[Kingsman-99]

Description

contracts/split/src/events.rs has no module-level documentation. New contributors adding events often use inconsistent symbol naming (e.g. some use symbol_short!, others use Symbol::new). A //! module doc at the top of the file that explains the naming convention, topic layout, and when to use next_seq would reduce inconsistencies.

Acceptance Criteria

• Add a //! # Event naming convention module-level doc comment at the top of contracts/split/src/events.rs
• The comment covers: topic format (split, <verb>, invoice_id), when to call next_seq, and when to use symbol_short! vs Symbol::new
• cargo doc builds without warnings
• cargo test passes

Context

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