test: bring MUSD and MuzixCatalog coverage up to repo bar

[Pattermesh]

Summary

MUSD.t.sol and MuzixCatalog.t.sol had only ~2 tests each, far below sibling suites (27–35). This PR brings both up to par.

• MUSD.t.sol: 2 → 28 tests
• MuzixCatalog.t.sol: 2 → 29 tests

What's covered

MUSD (src/MUSD.sol)

• Token metadata (name/symbol/decimals), constructor (catalog + owner wiring)
• mint happy path + onlyOwner revert (Ownable.OwnableUnauthorizedAccount)
• Vanilla ERC20 transfer/transferFrom + allowance accounting, with IERC6093 custom-error reverts (ERC20InsufficientBalance, ERC20InsufficientAllowance)
• transferWithRoyalty (atomic single distribution): pending-withdrawal crediting, escrow into the contract, event emission, accumulation across calls, single-recipient and partial-split (<100%) remainder behavior, no-split and insufficient-balance reverts
• batchRoyaltyDistribution: multi-token splits, aggregate BatchRoyaltyProcessed event, onlyOwner + mismatched-array reverts, empty-array no-op, atomic revert when any token lacks a split
• Pull-payment claimPayments: transfer + zeroing of pending, Withdrawal event, no-funds and double-claim reverts, per-recipient independence, claim across multiple distributions
• ERC20Permit (EIP-2612) allowance via signed permit

MuzixCatalog (src/MuzixCatalog.sol)

• Construction (name/symbol/owner)
• mintMusic: ownership, tokenURI, ISRC/artist metadata, monotonic token ids, onlyOwner revert, nonexistent-token tokenURI revert, transferability
• setRoyaltySplit: storage of recipients/shares, ERC-2981 10% pool royalty, and reverts for non-owner / array mismatch / total ≠ 100% / nonexistent token, plus overwrite
• fractionalize view (multi- vs single-recipient vs no split)
• depositRevenue / claimStreamingRevenue ETH streaming pull-payments: share-weighted payout, incremental claim after top-up, double-claim-without-new-revenue revert, non-stakeholder and zero-revenue reverts, anyone-can-fund
• ERC-2981 / ERC-721 / ERC-165 introspection via supportsInterface, and royaltyInfo zero before any split

Test infra

Uses a configurable MockCatalog for MUSD (depends only on the royaltySplits ABI slice) and a Stakeholder receiver harness for catalog streaming claims, matching the stub-based conventions in the existing sibling suites.

Verification

forge build   # Compiler run successful (exit 0)
forge test    # 161 passed, 0 failed, 0 skipped across 6 suites (exit 0)
forge fmt --check test/MUSD.t.sol test/MuzixCatalog.t.sol   # clean

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