feat(multipair): G10 multi-pair pipeline + USE_MULTIPAIR knob (Option C)

- prepare_hourly.py: parameterize PAIR env var; OUT_DEFAULT per-pair; load_m1_from_zips(pair=)
- prepare_multipair.py: inner-join 5-pair hourly parquets on datetime → wide parquet
  cols: datetime, {pair}_ret, {pair}_rv × n_pairs; eurusd_rv = target
- fetch_multipair.py: download GBPUSD/USDJPY/USDCHF/AUDUSD M1 2008-2023 from histdata
- train.py: USE_MULTIPAIR knob (JEPA_USE_MULTIPAIR=1); build() reads multipair parquet
  with n_channels = n_pairs × 2; target = eurusd_rv
- Taskfile: data:fetch:multipair, data:prepare:pair, data:prepare:multipair, data:test updated
- 7 new tests in test_multipair.py; 34/35 pass (1 SKIP until multipair parquet built)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/fetch_multipair.py

@@ -0,0 +1,48 @@
+"""Fetch G10 FX M1 data from histdata.com for all pairs except EURUSD (already fetched).
+
+Each pair's zips go into data/raw/{pair}/ to avoid collisions.
+Output: data/raw/gbpusd/DAT_ASCII_GBPUSD_M1_YYYY.zip etc.
+
+    python scripts/fetch_multipair.py
+    PAIRS=gbpusd,usdjpy YEARS=2020,2021 python scripts/fetch_multipair.py
+"""
+import os
+import time
+
+from histdata import download_hist_data
+from histdata.api import Platform as P, TimeFrame as T
+
+PAIRS_DEFAULT = ["gbpusd", "usdjpy", "usdchf", "audusd"]
+YEARS_DEFAULT = list(range(2008, 2024))
+
+
+def main():
+    pairs_env = os.environ.get("PAIRS", "")
+    pairs = [p.strip() for p in pairs_env.split(",")] if pairs_env else PAIRS_DEFAULT
+
+    years_env = os.environ.get("YEARS", "")
+    years = [int(y.strip()) for y in years_env.split(",")] if years_env else YEARS_DEFAULT
+
+    for pair in pairs:
+        out_dir = f"data/raw/{pair}"
+        os.makedirs(out_dir, exist_ok=True)
+        print(f"\n=== {pair.upper()} ===")
+        for yr in years:
+            out_path = os.path.join(out_dir, f"DAT_ASCII_{pair.upper()}_M1_{yr}.zip")
+            if os.path.exists(out_path):
+                print(f"  {yr} already present, skip")
+                continue
+            try:
+                f = download_hist_data(
+                    year=str(yr), month=None, pair=pair,
+                    platform=P.GENERIC_ASCII, time_frame=T.ONE_MINUTE,
+                    output_directory=out_dir,
+                )
+                print(f"  fetched {yr} → {f}")
+            except Exception as e:
+                print(f"  {yr} FAILED: {e}")
+            time.sleep(2)
+
+
+if __name__ == "__main__":
+    main()