backtest cleanup: drop four research/one-time scripts; books and stale cache moved out (owner decisions 2026-09-15)

Discussion-first cleanup of backtest/ and scripts/. Nothing in the signal
core or the backtester changed.

Removed (last commit containing them: 931b265):
- backtest/diagnose_filters.py + its two Module 18 tests
- scripts/shadow_scan.py (Phase 4 shadow scan, never run)
- scripts/backfill_news_archive.py, scripts/migrate_signal_log_news.py
  (one-time scripts; the interning test keeps its live-path half, the
  prune test goes)

Kept: the replay-recording stack (replay_record.py, replay_scorecard.py,
ff_calendar_scraper.py, cache/calendar.parquet, Module 33).

Local gitignored data was moved to ~/Development/ForexBot2_trash_2026-09-15
(346 MB) for the owner to delete: all 35 pinned backtest/*.csv books, the
three research output folders, the BTC/ETH cache snapshots, the Tiingo
parquets and 13 uncited Oanda windows. The three research-folder ignore
rules are dropped with the folders.

Verification: all five gate books regenerate offline from the pruned cache
with the recorded MD5s (M5 W2 29120a1e, M15 W2 d185f099, M15 OOS 6f8f24b4,
H1 W3 3b45d12b, H4 W3 e75adae1; NumPy CPU, 0 cache fallbacks). Suite
506 passed, 0 failed, 33 modules.

.gitignore

@@ -29,6 +29,3 @@ backtest/*.csv
 backtest/__pycache__/
 .env.*
 logs/
-backtest/research/sizing_2026-09/
-backtest/research/replay_2026-09/
-backtest/research/cycle_2026-09-10/