Backfill 13,770 FIGI identifiers for US equities

Source: Finnhub `/stock/symbol?exchange=US` endpoint (free tier, single
API call returns the full US listing universe with FIGI identifiers).
FIGI is Bloomberg's Open Financial Identifier — an ISO 6166-aligned
public identifier maintained via the OpenFIGI initiative.

Stats:
 * 13,769 figi cells filled (was: 18,114 empty in US rows)
 * 13,469 composite_figi cells filled
 * 13,043 shareclass_figi cells filled
 * 13,770 unique rows touched
 * Diff: 13,770 insertions / 13,770 deletions

Gating:
 * FIGI regex `^BBG[0-9A-Z]{9}$` (FIGI format)
 * Skip rows where the column is already populated (zero overwrite)
 * Match by exact symbol — no fuzzy matching, no name similarity

Why FIGI matters: FIGI is the only freely-licensed global financial
identifier. ISIN/CUSIP/SEDOL are paywalled standards; FIGI was created
specifically as the open alternative (Bloomberg/OpenFIGI). Filling FD's
FIGI columns makes downstream cross-reference (FIGI -> ISIN, FIGI ->
CUSIP, FIGI -> SEDOL) possible via OpenFIGI's free public API.

Markets explicitly NOT covered:
 * Non-US exchanges — Finnhub free tier returns 401 for /stock/symbol
   on non-US exchanges.
 * Tickers using "." instead of "-" (e.g. BRK.A vs BRK-A) — Finnhub's
   format diverges from FD's for a handful of share-class tickers;
   left for follow-up.

Author: dokson