refactor: class-based alpha factory + month-partitioned data pipeline

Replace the old signal/strategy/backtest modules with a decoupled
data → alpha → combo pipeline (parquet between phases, .pq extension).

Alphas:
- BaseAlpha + @register_alpha factory/plugin registry; one file per
  built-in (reversal, reversal_vol, momentum); external alphas via
  --alpha-module. Alphas are z-scored position weights, not predictors.

Data:
- baostock primary / akshare fallback, treated consistently.
- New --universe all (~5000 A-shares via query_all_stock, filtered).
- login-once batch downloader; empty-string OHLCV coerced to NaN.
- Month-partitioned dataset {output_dir}/{universe}/month=YYYY-MM/*.pq
  with chunked durability flushes; --data-path is the dataset dir.

CLI logs at INFO by default (--log-level) so progress is visible.
Docs (README, CLAUDE.md) updated incl. pipeline diagram and roadmap
TODOs for portfolio construction / backtest / paper trading.

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

pipeline/common/schema.py

@@ -0,0 +1,33 @@
+"""Column contracts for pipeline parquet files."""
+
+from typing import Final
+
+# Required columns for data parquet files (daily bars, alternative data, etc.)
+DATA_COLUMNS: Final[list[str]] = [
+    "symbol_id",   # str: internal code like 'sh600000'
+    "symbol_name", # str: stock name like '浦发银行'
+    "date",        # date
+    "open",        # float64
+    "high",        # float64
+    "low",         # float64
+    "close",       # float64
+    "volume",      # float64 (shares)
+    "amount",      # float64 (turnover in yuan)
+]
+
+# Required columns for alpha parquet files.
+# Alphas are position WEIGHTS: positive=long, negative=short.
+ALPHA_COLUMNS: Final[list[str]] = [
+    "symbol_id",   # str: matches DATA_COLUMNS symbol_id
+    "date",        # date: aligned with data dates
+    "alpha_name",  # str: identifies which alpha (e.g. 'reversal_5d')
+    "weight",      # float64: position weight, signed
+]
+
+# Required columns for combo parquet files.
+COMBO_COLUMNS: Final[list[str]] = [
+    "symbol_id",   # str
+    "date",        # date
+    "combo_name",  # str: identifies which combo (e.g. 'equal_weight')
+    "weight",      # float64: combined weight, signed
+]