Add test suite across all packages

[PlutoYork]

Zero test coverage currently. Need pytest suites for all five packages.

Priority areas

• hedonics/taxonomy.py — domain lookup, list
• hedonics/htc.py — HTC creation, code formatting
• hedonics/hqc.py — HQC creation, subcategory lookup, blockers
• hedonics/fungibility.py — exchange rate lookup, compute_exchanges(), priority sorting
• altpath/assessment.py — scoring, gaps, strengths, blocked_ends(), optimize()
• highnoon/assess.py — read_project_signals(), SoftwareAssessment
• mainstreet/datasources.py — ATUS profiles, BLS API (mock)
• mainstreet/policy.py — PolicyAssessment impacts, distributional effects
• frontpage/feed.py — score_item(), rank_feed(), explain_ranking()

Approach

• pytest + pytest-cov
• Mock external APIs (BLS) in tests
• CI via GitHub Actions