Add calendar research infra: FF scraper + verified event store

Scrapes Forex Factory weekly pages (epoch datelines, actual/forecast/
impact/betterWorse) into backtest/cache/calendar.parquet (gitignored;
rebuild with the scraper). Timestamps verified: NFP/CPI/ECB times exact
across DST, and high-impact event bars average 3.5x ATR with the spike
vanishing at +/-1h shifts. calendar_store provides precise_events and
event_window_mask for screens and counterfactuals.

backtest/research/__init__.py

@@ -0,0 +1,5 @@
+"""Research tooling: calendar sourcing/store and edge-screening utilities.
+
+Nothing here is imported by the live bot; these modules support
+backtesting and edge validation only.
+"""