feat: implement issues #559, #560, #561, #562

[adeniran19-maker]

Summary

Implements four feature issues in the Split Contract:

• Invoice Creator Revenue Share #559 Creator Revenue Share — adds creator_fee_bps to InvoiceOptions2/Invoice, validates the combined fee cap in _create_invoice_inner, deducts the creator fee in _release_full and _release_tranches, and emits creator_fee_paid events.
• Invoice Creator Migration #560 Creator Migration — adds nominate_new_creator / accept_creator_role entry points, pending_creator_key storage, and creator_nominated / creator_migrated events.
• Recipient Payout Ordering Guarantee #561 Payout Ordering — adds sort_recipients in calc.rs for canonical address ordering, and emits payout_initiated events with recipient index during release.
• Invoice Soft-Delete with Tombstone Record #562 Soft-Delete with Tombstone — adds Tombstone type, delete_invoice / get_tombstone entry points, tombstone_key storage, InvoiceDeleted / FundsUnclaimed / NotDeleted guards, and invoice_state_changed updates.

Files Changed

• contracts/split/src/types.rs
• contracts/split/src/error.rs
• contracts/split/src/events.rs
• contracts/split/src/storage_keys.rs
• contracts/split/src/calc.rs
• contracts/split/src/lib.rs
• contracts/split/src/test.rs

Testing

• Added tests for all four issues in test.rs.
• Note: upstream main currently has a pre-existing unclosed-delimiter compilation failure in lib.rs (claim_delayed_payout/pay bodies). This PR includes minimal structural fixes so the new feature code can compile, but those pre-existing issues are not the focus of this PR.

Closing Issues

Closes #559, Closes #560, Closes #561, Closes #562

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