Fix evidence bundle signature verification

[Mopati123]

Summary

• Align optional evidence bundle signing and verification on the same canonical payload.
• Sign deterministic bundle identity data after bundle_id, timestamp, and merkle_root are known.
• Preserve public bundle fields and to_dict() display truncation.
• Add focused regression coverage for emit-then-verify success and tamper rejection.

Validation

• python -m pytest tests/rootfile/test_evidence_bundle_signature.py -q -> 4 passed
• python -m pytest tests/rootfile -q -> 115 passed
• python -m pytest tests -q -> 208 passed, 9 warnings
• python -c "from tachyonic_chain.audit_log import verify_execution_evidence_chain; print(verify_execution_evidence_chain('logs/execution_evidence.jsonl').valid)" -> True
• git diff --cached --check -> passed

Safety

• No broker execution or canary run in this pass.
• Runtime logs/data, .env, .github drift, and runner drift were not included.
• kluster was attempted after edits, but review is blocked because the trial has ended.