Switch active data source to Oanda v20 (practice feed)

config.json.example: data_providers.active = "oanda" (credentials stay
in env/.env or the top-level oanda block; TD and Tiingo remain
registered fallbacks). Backtest parquet cache is now keyed per provider
— Oanda and TD bars diverge (2026-07-24 study), so a feed switch must
never serve another feed's cached bars; twelve_data keeps the legacy
un-prefixed filename so pinned-book caches stay valid. 254 tests green;
live smoke-tested against the practice API (66 contract checks: all
scanned TFs, boundary/freshness guarantees, real spread $0.71,
fetch_range pagination, typed error paths, CLI + backtest end-to-end).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mc7XKYyPySVXiUpcN82QEK

config.json.example

@@ -6,10 +6,11 @@
     "alert_min_quality": "C"
   },
   "data_providers": {
-    "active": "twelve_data",
+    "active": "oanda",
     "twelve_data": {
       "api_key": "REPLACE_WITH_YOUR_TWELVE_DATA_API_KEY"
     },
+    "oanda": {},
     "tiingo": {
       "api_key": "REPLACE_WITH_YOUR_TIINGO_API_KEY_OR_EMPTY"
     }