IB20: add from/to/amount/nonce to Memo event

[eric-ships]

Motivation

The previous Memo(bytes32 indexed memo) event required indexers to correlate it to the preceding Transfer log via logIndex - 1. This coupling is fragile: any log interleaved between Transfer and Memo breaks the join silently. Even with from/to added, the identity is not unique — the same sender can pay the same recipient the same amount with the same memo multiple times (retries, repeated invoices).

The new signature is fully self-contained and unconditionally unique:

Memo(address indexed from, address indexed to, uint256 amount, bytes32 indexed memo, uint256 nonce)

nonce is a per-token monotonic counter that increments on every *WithMemo call, guaranteeing uniqueness even when (from, to, amount, memo) repeats. No join to Transfer is needed; indexers can consume Memo alone.

What changed

• IB20.sol: Memo event gains from, to, amount, nonce; natspec updated
• MockB20Storage: memoNonce uint256 appended at MEMO_NONCE_OFFSET = 15
• MockB20: all four *WithMemo functions read-increment-emit the nonce
• Memo test files: vm.expectEmit updated to assert the full new signature

What was tried / considered

Adding only from/to was considered but rejected: (from, to, memo) is still not unique if the same invoice is retried. Adding amount was also considered but (from, to, amount, memo) is similarly non-unique. A contract-level nonce is the only mechanism that gives unconditional uniqueness without relying on log position.

No ERC is broken: Transfer (ERC-20) is unchanged. Memo is a custom extension not defined by any standard.