fix(backtest): settle delisted US holdings (engine v3), PIT membership before 2011, registry causality test

- execution/trader: a held US stock with no print for 5 sessions that has left
  the index is sold at its last print (execution.SETTLE_AFTER_SESSIONS), in both
  the batch replay and the live session ledger. Acquired names used to stay at
  their last mark forever: lowrisk_top20 ended 2017-12..2026-10 with 11 of 26
  dead positions, and CAGR rises 1.7-5.5pp once they settle. ENGINE_VERSION
  cash-ledger-pit-v3; the API cache key follows.
- universe_history: current members whose pre-2011 addition the Wikipedia change
  log missed now start at the list page's "Date added" (80 of 94 current members
  added 2001-2010 were open-start, e.g. ISRG, MA, BKNG). Membership from 2016 on
  is unchanged; the research panel cache now keys on the membership intervals.
- research/lab/registered: pure-ETF strategies replay on the ETF panel (they ran
  as all-cash books or crashed); prepare_strategy refuses an ETF strategy that
  lacks its ETF universe.
- research/lab/factors: trend_hzz uses a month's regression betas only from the
  session its forward returns end (short months read 1-2 sessions ahead).
- tests: every registered strategy (US and CN) is truncation-invariant on the
  production signal path; batch and live ledgers settle a delisting identically.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

research/lab/panel.py

@@ -10,6 +10,7 @@ Panels are cached in data/research/*.panel.pkl; delete them after refetching.
 
 from __future__ import annotations
 
+import hashlib
 import json
 import os
 import pickle
@@ -103,11 +104,18 @@ def _tiingo_sig() -> list:
     return [len(files), round(mtime, 3)]
 
 
+def _membership_sig() -> str:
+    with open(os.path.join("data", "sp500_history.json")) as fh:
+        intervals = json.load(fh)["intervals"]
+    return hashlib.sha256(json.dumps(intervals, sort_keys=True).encode()).hexdigest()[:16]
+
+
 def load_us() -> Panel:
-    """Cached US panel, rebuilt whenever the Tiingo overlay changes."""
+    """Cached US panel, rebuilt whenever the Tiingo overlay or the S&P 500
+    membership intervals change."""
     path = os.path.join(RAW_DIR, "us.panel.pkl")
     key = os.path.join(RAW_DIR, "us.panel.key")
-    sig = _tiingo_sig()
+    sig = _tiingo_sig() + [_membership_sig()]
     if os.path.exists(path) and os.path.exists(key):
         with open(key) as fh:
             if json.load(fh) == sig: