Add signed evidence bundle file attestation

[cybertechsoft]

Adds signed SHA-256 file attestation to Evidence Bundle v2 so exported
human-readable and verification artifacts are cryptographically bound to
the bundle.

This change:

• covers exported bundle files with SHA-256 digests
• signs the canonical bundle attestation with the configured LoopGrid signer
• rejects modified, missing, or unexpected attested files
• preserves existing ledger and checkpoint semantics
• keeps legacy unattested Evidence Bundle v2 exports verifiable with an
  explicit legacy_unattested status
• adds regression tests for bundle-file tampering

Local validation completed:

• 58 tests passed
• clean bundle verification passed
• tampered report.html correctly rejected
• tamper verification returned exit code 2
• Docker build passed
• Docker /health passed
• Docker /ready passed
• Docker quickstart passed
• Docker-generated evidence independently verified