test(taproot): prove tampered prevout rejection

[BitHighlander]

Summary

• add a mixed Taproot/legacy negative test with a one-satoshi host amount mismatch
• require the exact firmware rejection before any signature is returned
• add the security case to the canonical firmware PDF catalog

SOP

This is test/report work only. It targets the upstream staging branch; nothing is published. The fork firmware candidate will pin the exact canonical merge commit after this PR is green and merged.

Validation

• python3 -m py_compile tests/test_msg_signtx_taproot.py scripts/generate-test-report.py
• git diff --check
• full device/emulator proof is intentionally delegated to the firmware candidate's Docker/GitHub CI PDF