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/errors.py

@@ -0,0 +1,23 @@
+"""Typed exceptions for the BLS client.
+
+Lets callers distinguish "you're throttled" from "your request was bad" — the
+two failure modes the old generic RuntimeError conflated.
+"""
+
+
+class BLSError(RuntimeError):
+    """Base class for all BLS API errors."""
+
+    def __init__(self, message, *, status=None, messages=None):
+        super().__init__(message)
+        self.status = status            # the API "status" field, e.g. REQUEST_FAILED
+        self.messages = messages or []  # the API "message" list, verbatim
+
+
+class BLSRequestError(BLSError):
+    """The API rejected the request (malformed series ID, bad year range, etc.)."""
+
+
+class BLSQuotaError(BLSError):
+    """The daily query threshold was exceeded (500/day for a registered key,
+    25/day unregistered). Resets at midnight Eastern."""