Fix broken series-ID builders; env-var config; project README

Repair every dead series-ID encoding (library now 82/82 query helpers
return live data, verified against the BLS API):

- JOLTS: 21-char format (was 18) — add state/area/sizeclass fields
- OES: national area code 0000000 (was invalid 0000400)
- ECI: correct owner/component/estimate encoding, default unadjusted (CIU)
- Productivity: 4-digit sector + 4-digit measure codes (was 2+3)
- QCEW: 13-char timeseries-API form (ENUUS00010510 / ENU{fips}00010{own}10)
- PPI: repoint finished-goods -> final demand (WPUFD4); keep alias
- ECEC: drop fabricated health-insurance/retirement helpers; add total benefits
- wages SOC: software developers 151132 -> 151252 (2018 SOC)

Tooling/docs:
- config reads BLS_API_KEY env var (takes precedence; config.py gitignored)
- add requirements.txt
- rewrite README as project front door + coverage table + limitations
- correct JOLTS/OES tables in series_id_formats.md, USAGE.md, dataset explorer

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bls_dataset_explorer.py

@@ -48,7 +48,7 @@ DATASETS = [
 
     ("JT", "Employment & Labor Force", "active",
      "Job Openings & Labor Turnover (JOLTS)",
-     "JTS000000000000JOL",
+     "JTS000000000000000JOL",
      "Monthly job openings, hires, quits, layoffs, and total separations "
      "by industry. Key measure of labor demand."),
 
@@ -134,7 +134,7 @@ DATASETS = [
     # ── Wages & Compensation ────────────────────────────────────────────────
     ("OE", "Wages & Compensation", "active",
      "Occupational Employment & Wage Statistics (OEWS)",
-     "OEUN000040000000000000001",
+     "OEUN000000000000000000001",
      "Annual employment and wage estimates for ~800 occupations "
      "at national, state, and metro levels."),