Add QCEW CSV module, ECEC breakdown, response caching + typed errors

Item 1 — QCEW county/industry detail:
- new bls_client/qcew.py: thin client over the QCEW Open Data CSV service
  (area/industry/size slices; no API key, no quota), with filter_rows helper
- covers the county/industry detail the timeseries API can't reach

Item 2 — ECEC benefit breakdown:
- new series.ecec() builder from the authoritative cm.estimate decode table
- real helpers: wages, total benefits, paid leave, supplemental pay,
  health insurance, retirement & savings, legally required + ecec_dashboard()
- FIX: ecec_total_benefits was CMU1036... (education/health industries only);
  correct all-civilian total benefits is CMU1030000000000D

Item 3 — caching + typed errors:
- bls_client/cache.py FileCache (on-disk, TTL); BLSClient(cache=True),
  queries_used counter (cache hits don't spend quota)
- bls_client/errors.py: BLSError / BLSQuotaError / BLSRequestError; quota
  exhaustion now raises a clear BLSQuotaError instead of a generic RuntimeError

Tests: 119 offline (+24) incl. ECEC goldens, QCEW fixture parse, cache/quota
behavior; live smoke extended to ECEC + QCEW. Helper sweep: 96/96 live.
README: caching/QCEW usage, updated coverage + limitations.

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

tests/test_client_cache.py

@@ -0,0 +1,83 @@
+"""Offline tests for caching, the query counter, and typed errors.
+
+A fake transport stands in for requests.post so nothing touches the network.
+"""
+import pytest
+
+import bls_client.client as client_mod
+from bls_client import BLSClient, BLSQuotaError, BLSRequestError
+from bls_client.cache import FileCache
+
+
+class FakeResp:
+    def __init__(self, payload):
+        self._payload = payload
+
+    def raise_for_status(self):
+        pass
+
+    def json(self):
+        return self._payload
+
+
+def _ok(series_id):
+    return {
+        "status": "REQUEST_SUCCEEDED",
+        "Results": {"series": [{"seriesID": series_id, "data": [{"year": "2024", "period": "M01", "value": "1.0"}]}]},
+    }
+
+
+def test_query_counter_and_cache(tmp_path, monkeypatch):
+    calls = {"n": 0}
+
+    def fake_post(url, json=None, timeout=None):
+        calls["n"] += 1
+        return FakeResp(_ok(json["seriesid"][0]))
+
+    monkeypatch.setattr(client_mod.requests, "post", fake_post)
+
+    c = BLSClient("key", cache=FileCache(cache_dir=tmp_path, ttl=86400))
+    c.fetch(["CES0000000001"], 2024, 2024)
+    assert c.queries_used == 1 and calls["n"] == 1
+
+    # identical request -> served from cache, no new network call
+    c.fetch(["CES0000000001"], 2024, 2024)
+    assert c.queries_used == 1 and calls["n"] == 1
+
+    # different request -> one more call
+    c.fetch(["CES0000000001"], 2023, 2024)
+    assert c.queries_used == 2 and calls["n"] == 2
+
+
+def test_no_cache_by_default(monkeypatch):
+    def fake_post(url, json=None, timeout=None):
+        return FakeResp(_ok(json["seriesid"][0]))
+
+    monkeypatch.setattr(client_mod.requests, "post", fake_post)
+    c = BLSClient("key")
+    assert c.cache is None
+    c.fetch(["CES0000000001"], 2024, 2024)
+    c.fetch(["CES0000000001"], 2024, 2024)
+    assert c.queries_used == 2   # no caching -> both hit the (fake) network
+
+
+def test_quota_error(monkeypatch):
+    def fake_post(url, json=None, timeout=None):
+        return FakeResp({
+            "status": "REQUEST_NOT_PROCESSED",
+            "message": ["Query exceeds the threshold of 500 queries per day."],
+        })
+
+    monkeypatch.setattr(client_mod.requests, "post", fake_post)
+    with pytest.raises(BLSQuotaError) as exc:
+        BLSClient("key").fetch(["CES0000000001"], 2024, 2024)
+    assert exc.value.status == "REQUEST_NOT_PROCESSED"
+
+
+def test_request_error(monkeypatch):
+    def fake_post(url, json=None, timeout=None):
+        return FakeResp({"status": "REQUEST_FAILED", "message": ["invalid series"]})
+
+    monkeypatch.setattr(client_mod.requests, "post", fake_post)
+    with pytest.raises(BLSRequestError):
+        BLSClient("key").fetch(["BOGUS"], 2024, 2024)