docs(5288): spec script-witness parity for Transaction.Ledger
Document the parity contract between the legacy `mkUnsignedTx` (cardano-api builder) and the extended `Transaction.Ledger` builder for native-script inputs, staking script, mint/burn script sources, and output reference scripts. Byte-equivalence on six representative scenarios is the acceptance criterion; non-script behaviour and `ledgerMintPlumbingSpec` invariants from #5287 remain green. `mkUnsignedTransaction` itself is explicitly out of scope (tracked by #5285), as are signing (#5289), Dijkstra-era extensions (#5209), and cardano-api dependency removal (#5290).