fix: omit unmeasured receipt latency

[stackbilt-admin]

Summary

• make latency percentiles optional in EvaluationReceipt public aggregates
• let native frameworks omit measurements they do not collect
• document that zero must not stand in for unknown evidence
• bump @stackbilt/evals to 0.2.2

Verification

• npm test (12 tests)
• npm run build
• git diff --check