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/series.py

@@ -336,6 +336,55 @@ def eci(
     return f"CI{adj}{owner}{component}{'0'*9}{estimate}"
 
 
+# ---------------------------------------------------------------------------
+# ECEC — Employer Costs for Employee Compensation
+# Component ("estimate") codes from the BLS cm.estimate decode table.
+# ---------------------------------------------------------------------------
+_ECEC_COMPONENT = {
+    "total_compensation":     "01",
+    "wages_and_salaries":     "02",
+    "total_benefits":         "03",
+    "paid_leave":             "04",
+    "supplemental_pay":       "09",
+    "insurance":              "13",
+    "health_insurance":       "15",
+    "retirement_and_savings": "18",
+    "legally_required":       "21",
+    "workers_compensation":   "27",
+}
+
+_ECEC_OWNER = {
+    "civilian": "1",
+    "private":  "2",
+    "state_local_gov": "3",
+}
+
+def ecec(
+    component: str = "total_compensation",
+    owner:     str = "civilian",
+    datatype:  str = "D",      # D = cost per hour worked (dollars)
+) -> str:
+    """
+    ECEC series — employer cost per hour worked, all industries / all workers.
+
+    Format: CM + U + owner(1) + component(2) + 10 zeros + datatype(1) = 17 chars.
+
+    Args:
+        component: key in _ECEC_COMPONENT (e.g. "health_insurance") or a raw 2-digit code
+        owner:     "civilian" | "private" | "state_local_gov" (or raw "1"/"2"/"3")
+        datatype:  "D" = cost per hour in dollars (default); "P" = percent of total comp
+
+    Examples:
+        ecec()                              → "CMU1010000000000D"  (civilian total compensation)
+        ecec("health_insurance")            → "CMU1150000000000D"
+        ecec("retirement_and_savings")      → "CMU1180000000000D"
+        ecec("total_benefits", "private")   → "CMU2030000000000D"
+    """
+    own  = _ECEC_OWNER.get(owner, owner)
+    code = _ECEC_COMPONENT.get(component, component)
+    return f"CMU{own}{code}{'0'*10}{datatype}"
+
+
 # ---------------------------------------------------------------------------
 # Productivity (Major Sector)
 # ---------------------------------------------------------------------------