fix(findings): stabilize evidence signatures

[rohitjavvadi]

Summary

This makes fallback finding signatures deterministic when two provider results describe the same evidence with different object key order or evidence array order. The signature still changes when the evidence content changes.

Fixes #83.

Verification

• Added focused regression coverage for stable evidence signatures
• Ran the focused findings test
• Ran the full test suite
• Ran typecheck, lint, format check, diff check, and build