isWellFormed

Validates that this dust spend is well-formed.

Validation Checks:

• Fee amount is positive

• Old nullifier is 64 hex characters (32 bytes)

• New commitment is 64 hex characters (32 bytes)

• Proof is present (if required)

Note: This is basic format validation. Full validation requires:

• Checking nullifier hasn't been used before

• Verifying the zero-knowledge proof

• Checking Merkle tree inclusion

• Validating against on-chain state