What is the OpCat opcode in Kaspa's KIP17?

OpCat is a new opcode (0x7e) introduced in Kaspa's KIP17 that concatenates two byte strings inside a script. Alongside OpSubstr (0x7f), which extracts substrings, and transaction introspection opcodes, OpCat is a fundamental building block for covenant constructions — scripts that control how a coin can be spent. It works by letting a script reconstruct raw transaction bytes from data the spender provides and then hash them internally. For a beginner, this matters because it means a Kaspa script can now inspect and verify the structure of the transaction spending a coin, not just check a signature.