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>

tests/test_series_builders.py

@@ -0,0 +1,88 @@
+"""Unit tests for the low-level series-ID builders in bls_client.series.
+
+These lock the exact encodings against known-good BLS series IDs. They run
+offline (no network) and are the regression guard for the encoding bugs that
+were fixed in June 2026 (JOLTS width, OES area code, ECI component/estimate,
+productivity sector/measure widths, QCEW form).
+"""
+import pytest
+
+from bls_client import series as S
+
+# (callable, kwargs) -> exact expected series ID. Every value here has been
+# confirmed to return data from the live BLS API.
+GOLDEN = [
+    # LAUS — 20 chars
+    (S.laus_state,  dict(state_fips=11, measure="rate"),                 "LAUST110000000000003"),
+    (S.laus_state,  dict(state_fips=11, measure="rate", seasonal=True),  "LASST110000000000003"),
+    (S.laus_county, dict(state_fips=11, county_fips=1, measure="rate"),  "LAUCN110010000000003"),
+    (S.laus_msa,    dict(state_fips=11, cbsa_code=47900, measure="rate"),"LAUMT114790000000003"),
+    # CES national / state
+    (S.ces_national, dict(),                                             "CES0000000001"),
+    (S.ces_national, dict(industry="manufacturing"),                    "CES3000000001"),
+    (S.ces_state,    dict(state_fips=11),                               "SMS11000000000000001"),
+    # CPI / PPI
+    (S.cpi,          dict(),                                             "CUUR0000SA0"),
+    (S.cpi,          dict(item="core", seasonal=True),                  "CUSR0000SA0L1E"),
+    (S.ppi_commodity,dict(),                                            "WPU00000000"),
+    # OES — 25 chars, national area 0000000 (was the 0000400 bug)
+    (S.oes_national, dict(),                                            "OEUN000000000000000000001"),
+    (S.oes_national, dict(occupation_code="151252", data_type="annual_median"),
+                                                                        "OEUN000000000000015125213"),
+    # JOLTS — 21 chars / 15 zeros (was 18 chars / 12 zeros)
+    (S.jolts, dict(),                                                   "JTS000000000000000JOL"),
+    (S.jolts, dict(element="quits", rate_level="R"),                    "JTS000000000000000QUR"),
+    (S.jolts, dict(element="hires", seasonal=False),                    "JTU000000000000000HIL"),
+    # ECI — 17 chars, unadjusted, component 10/20/30, estimate A
+    (S.eci, dict(),                                                     "CIU1010000000000A"),
+    (S.eci, dict(component="20"),                                       "CIU1020000000000A"),
+    (S.eci, dict(component="30"),                                       "CIU1030000000000A"),
+    (S.eci, dict(owner="20"),                                           "CIU2010000000000A"),
+    # Productivity — 11 chars, 4-digit sector + 4-digit measure
+    (S.productivity, dict(),                                            "PRS85006092"),
+    (S.productivity, dict(sector="manufacturing", measure="unit_labor_cost"), "PRS30006112"),
+    (S.productivity, dict(sector="business"),                          "PRS84006092"),
+]
+
+
+@pytest.mark.parametrize("fn,kwargs,expected", GOLDEN, ids=[f"{f.__name__}-{e}" for f, _, e in GOLDEN])
+def test_builder_exact_id(fn, kwargs, expected):
+    assert fn(**kwargs) == expected
+
+
+# --- Format invariants (catch width regressions even for un-goldened inputs) ---
+
+def test_jolts_is_21_chars():
+    for elem in ("job_openings", "hires", "quits", "layoffs", "total_separations"):
+        sid = S.jolts(elem)
+        assert len(sid) == 21, f"{sid} should be 21 chars"
+        assert sid.startswith("JTS")
+
+
+def test_oes_is_25_chars_national():
+    sid = S.oes_national()
+    assert len(sid) == 25
+    assert sid.startswith("OEUN0000000"), "national area code must be 0000000"
+
+
+def test_eci_is_17_chars_and_unadjusted_by_default():
+    sid = S.eci()
+    assert len(sid) == 17
+    assert sid.startswith("CIU"), "12-month % change ECI is published unadjusted (CIU)"
+
+
+def test_productivity_is_11_chars():
+    sid = S.productivity()
+    assert len(sid) == 11
+    assert sid.startswith("PRS")
+
+
+def test_laus_state_is_20_chars():
+    assert len(S.laus_state(48, "rate")) == 20
+
+
+def test_seasonal_flag_flips_adjustment_char():
+    assert S.cpi(seasonal=False).startswith("CUU")
+    assert S.cpi(seasonal=True).startswith("CUS")
+    assert S.jolts(seasonal=False).startswith("JTU")
+    assert S.jolts(seasonal=True).startswith("JTS")