feat: add co-creator revenue splits (#516)

[nomsoscript]

Closes #516

Changes

• Added optional immutable CoCreatorConfig support during creator registration.
• Validates co-creator share_bps to require 1..=9999.
• Splits creator fees on buy and sell between the creator fee recipient and co-creator.
• Added get_co_creator and get_co_creator_fee_balance read methods.
• Emits CoCreatorFeeEarned when a co-creator receives a fee.
• Added focused tests for valid splits, invalid shares, events, config reads, and default no-co-creator behavior.

Testing

• git diff --check