refactor(catalog): retire royalty splits from MuzixCatalog now that Labelton owns them

[Pattermesh]

Summary

Once LABELTON is merged and owns the cap-table, MuzixCatalog's royalty-split surface is redundant and has known footguns. Retire it.

Concrete changes

• Remove _royaltySplits, setRoyaltySplit, royaltySplits(uint256), claimStreamingRevenue, totalStreamingRevenue, claimedBalance, depositRevenue from MuzixCatalog.
• Fix or remove the ERC-2981 default _setTokenRoyalty(tokenId, address(this), 1000) — funds currently accumulate at the contract with no withdrawal path.
• Decide whether to retire MuzixCatalog entirely (delete) or repurpose as lightweight artwork / release-art NFT with no royalty surface.

Footguns this closes

• ERC-2981 default sends 10% to address(this) with no withdrawal.
• setRoyaltySplit lets a transferred token's new owner redirect cash flows unilaterally.
• ISRC stored as free-form string with no uniqueness.

Migration

If MuzixCatalog tokens already exist on testnet/mainnet, plan a migration step — for each existing token, register a LABELTON master with the equivalent cap-table.

Depends on

Part of #38. Depends on companion Labelton.sol PR + #C (MUSD wired to Labelton).

cc @abhicris