feat: phase 1 — data downloader, backtrader runner, SMA strategy

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

strategies/base.py

@@ -0,0 +1,23 @@
+"""Base strategy and example SMA crossover for Chinese equities."""
+import backtrader as bt
+
+
+class SmaCross(bt.Strategy):
+    """Simple SMA crossover strategy: buy when fast crosses above slow, sell when below."""
+
+    params = (
+        ("fast", 10),
+        ("slow", 30),
+    )
+
+    def __init__(self):
+        self.fast_ma = bt.indicators.SMA(self.data.close, period=self.params.fast)
+        self.slow_ma = bt.indicators.SMA(self.data.close, period=self.params.slow)
+        self.crossover = bt.indicators.CrossOver(self.fast_ma, self.slow_ma)
+
+    def next(self):
+        if not self.position:
+            if self.crossover > 0:  # fast crosses above slow
+                self.buy()
+        elif self.crossover < 0:  # fast crosses below slow
+            self.close()