Add unit test coverage for assessment.ts, presentation.ts, and cross-contract derivation

[xhulz]

Currently these modules are only exercised through integration tests (pipeline.test.ts).

Missing direct unit tests:

• assessment.ts: buildAttackAssessment() edge cases (upgrade paths, cross-contract signals)
• presentation.ts: describeDecision(), describeConfirmationStatus(), describeVerdictSummary()
• orchestrator.ts: deriveCrossContractFindings() in isolation

Adding these would let contributors verify behavior without running the full pipeline.