Sic Naics Coverage

SIC/NAICS coverage stats — what % of entities have codes populated.

Operational visibility for backfill progress and ongoing classifier coverage. Per-granularity counts so we can see the always-2-digit invariant in practice (2-digit should approach the populated total; 4-digit and 6-digit may be lower due to ambiguous_deep rows where deeper digits are intentionally null per ADR-008).

Returns: { "state": "CO" | "ALL", "total": 71744, "coverage": { "sic_2_digit": {"count": 12345, "pct": 17.2}, "sic_4_digit": {"count": 12000, "pct": 16.7}, "naics_2_digit": {"count": 12345, "pct": 17.2}, ... } }