Add pyproject.toml packaging and pytest suite

- pyproject.toml: setuptools build, deps (requests), dev extra (pytest),
  pytest config with opt-in `live` marker
- tests/: 95 offline tests locking every series-ID encoding against
  known-good IDs (regression guard for the JOLTS/OES/ECI/productivity/QCEW
  fixes), plus a live API smoke test behind `-m live`
- README: install-as-package + test instructions; drop the now-resolved
  "no packaging"/"no tests" limitations
- gitignore build/test artifacts

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

README.md

@@ -26,6 +26,8 @@ All 82 zero-argument query helpers are verified against the live API.
 
 ```bash
 pip install -r requirements.txt        # just `requests`
+# or install the package itself (editable):
+pip install -e .
 ```
 
 ## Configure your API key
@@ -48,6 +50,14 @@ python3 examples/basic_pull.py         # payrolls, CPI dashboard, DC-region unem
 python3 examples/custom_series.py      # building custom series IDs
 ```
 
+## Tests
+
+```bash
+pip install -e ".[dev]"                # pytest
+pytest                                 # offline: 95 tests locking the series-ID encodings
+pytest -m live                         # also hit the live API (needs network + BLS_API_KEY)
+```
+
 ---
 
 ## What's in the box
@@ -100,10 +110,6 @@ for the series-ID decode tables.
   benefits totals are currently exposed.
 - **No caching / rate-limit handling.** Repeated runs spend against the 500/day quota; a
   small on-disk cache and a friendly error on `REQUEST_NOT_PROCESSED` (quota hit) would help.
-- **No packaging.** Importable in-tree but not `pip install`-able; add a `pyproject.toml`
-  to ship it as a real package.
-- **No automated tests.** A pytest suite asserting builder outputs against known-good IDs
-  (and a live smoke test behind a marker) would lock the series-ID encodings in place.
 
 ---