test: edge cases for salary-compare and offer-compare helpers

[aryanB1706]

Summary

Adds node tests for the honest-comp helpers (web/lib/salary-compare.mjs, web/lib/offer-compare.mjs) — closes #24.

What's covered

• Partial bands: min-only / max-only / both-empty for target and posted bands
• Currency: missing defaults to USD; mismatch stays labeled per row, no silent conversion
• Offer compare: 2 roles with structured fields vs unstructured notes (notes never mined for numbers)
• Guard: helpers never fabricate a market average (gap kinds/labels + rows keep exact input bounds)

Verification

• npm run test:career-os green
• Full npm test green
• Test-only change; no module behavior or UI touched
• Fixtures are fictional