fix(screen): validate ETF subtype taxonomy

[bearmug]

Summary

• screen_etf now filters on type=fund AND subtype=etf upstream and post-filters client-side
• Returns etfClassification: "verified" only for confirmed ETF subtypes
• Exposes type, subtype, and expense_ratio (explicit null when missing) in default columns
• Preferred shares, corporate instruments, CEFs, mutual funds, and trusts are excluded/labeled
• Retrieval metadata (timestamp, source endpoint, cache-hit provenance) preserved from 0.7.0
• Deterministic unit fixtures + TV_INTEGRATION-gated live regression fixture
• Documents known-symbol lookup fallback when subtype cannot be verified

Closes #60

Test Plan

• Unit suite: 252/252 pass
• npm run build (tsc) passes
• Integration fixture compiles and is correctly gated behind TV_INTEGRATION=1