[Jimbo4350]

Context

Sub-transactions currently require all signing keys to be supplied to sign together. Add Dijkstra sub-transaction witness to create an individual key witness and sub-transaction assemble to combine an unsigned sub-transaction with separately collected witness files.

Both commands reuse the existing Dijkstra witness format and preserve the body bytes and script payloads. They reject Byron keys/bootstrap witnesses, and assembly rejects witnesses from other eras.

How to trust this PR

cardano-cli dijkstra sub-transaction witness --sub-tx-file sub.unsigned --signing-key-file payment.skey --out-file payment.witness
cardano-cli dijkstra sub-transaction assemble --sub-tx-file sub.unsigned --witness-file payment.witness --out-file sub.signed

Validation completed:

• Built the CLI and both test executables.
• All 41 selected sub-transaction tests passed, including equivalence with direct signing, empty/multiple/duplicate witnesses, preservation of canonical and noncanonical body bytes and script payloads, and invalid-input rejection.
• All 628 selected golden/help tests passed, including the witness/assemble/embed pipeline and existing transaction witnessing/assembly coverage.
• Fourmolu and HLint passed. The repository's style-check script also passed; the installed commit-hook launcher references an unavailable Nix executable.

Changelog

- description: Add Dijkstra sub-transaction witness and assemble commands for separate key witnessing and assembly.
  type:
    - feature

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• Self-reviewed the diff
• Changelog fragment added in .changes/