docs: add data/README.md — download instructions and split definitions

data/README.md

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+# Data
+
+Raw and processed FX data is **gitignored** — never committed.
+
+## Download: DUKASCopy G10 tick data
+
+Use the provided fetch task:
+
+```bash
+task data:fetch    # downloads raw tick data → data/raw/
+task data:process  # resamples to hourly OHLCV + features → data/processed/
+task data:validate # gap detection, outlier report, regime coverage check
+```
+
+## Expected structure (local only)
+
+```
+data/
+  raw/          G10 pairs, tick OHLCV, 2003–present (gitignored)
+  processed/    Hourly log-returns, rolling HV, VIX-merged (gitignored)
+  cache/        Intermediate artefacts (gitignored)
+  config.yaml   Download configuration (committed)
+```
+
+## Pairs
+
+EUR/USD, GBP/USD, USD/JPY, USD/CHF, AUD/USD, USD/CAD, NZD/USD, EUR/GBP, EUR/JPY, EUR/CHF
+
+## Splits
+
+| Split | Date range | Purpose |
+|---|---|---|
+| Train | 2008-01-01 – 2022-12-31 | Model training only |
+| Held-out test | 2023-01-01 – 2023-12-31 | Final evaluation — do not open until evaluation |
+
+**Training cutoff is hard: 2023-01-01. No post-2022 data informs any design decision.**