HWI parity: cryptographically validate signatures and signed PSBTs

[trevarj]

Goal

Strengthen HWI parity tests for nondeterministic signatures without requiring byte-for-byte equality.

Current state

• BitBox02 signmessage checks the JSON shape and that the base64 payload is 65 bytes, then skips exact comparison with the reference signature.
• signtx compares the signed flag and unsigned transaction and checks that an expected partial-signature key exists.
• Those checks can accept malformed signatures or meaningful PSBT differences.

Scope

• Cryptographically verify each signmessage signature against the expected message and device public key.
• Verify every expected PSBT signature against the correct transaction sighash, input, and public key.
• Compare meaningful PSBT fields and metadata while excluding only genuinely nondeterministic signature encodings.
• Apply the stronger helpers across Ledger, Jade, Coldcard, and BitBox02 parity cases.
• Add negative unit tests proving malformed or wrong-key signatures are rejected.

Acceptance criteria

• BitBox02 signmessage parity validates the signature instead of checking only length.
• Every successful signtx parity case validates the device signature cryptographically.
• Reference and candidate PSBTs are compared semantically outside nondeterministic signature bytes.
• Corrupted, wrong-message, wrong-input, and wrong-key fixtures fail focused tests.
• All four focused parity runners and upstream final gates remain green.
• Required non-emulator checks pass.