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>

bls_client/cache.py

@@ -0,0 +1,49 @@
+"""A tiny on-disk response cache for BLS requests.
+
+BLS data updates monthly/quarterly, so caching identical requests avoids
+re-spending the 500/day quota on reruns (reports, the explorer, test sweeps).
+Pure stdlib — no extra dependency.
+"""
+import hashlib
+import json
+import os
+import time
+from pathlib import Path
+
+
+class FileCache:
+    """JSON file cache keyed by request payload, with a TTL (seconds)."""
+
+    def __init__(self, cache_dir=None, ttl=86400):
+        self.dir = Path(cache_dir or os.path.expanduser("~/.cache/bls"))
+        self.ttl = ttl
+        self.dir.mkdir(parents=True, exist_ok=True)
+
+    @staticmethod
+    def key(payload: dict) -> str:
+        """Deterministic key from a request payload (API key excluded, series sorted)."""
+        relevant = {k: v for k, v in payload.items() if k != "registrationkey"}
+        if isinstance(relevant.get("seriesid"), list):
+            relevant["seriesid"] = sorted(relevant["seriesid"])
+        blob = json.dumps(relevant, sort_keys=True, default=str)
+        return hashlib.sha256(blob.encode()).hexdigest()
+
+    def _path(self, key: str) -> Path:
+        return self.dir / f"{key}.json"
+
+    def get(self, key: str):
+        p = self._path(key)
+        if not p.exists():
+            return None
+        if self.ttl is not None and (time.time() - p.stat().st_mtime) > self.ttl:
+            return None
+        try:
+            return json.loads(p.read_text())
+        except (OSError, ValueError):
+            return None
+
+    def set(self, key: str, value) -> None:
+        try:
+            self._path(key).write_text(json.dumps(value))
+        except OSError:
+            pass  # caching is best-effort; never fail a fetch over it