feat: add portfolio phase — discretize alpha weights into tradable positions

Adds a fourth pipeline phase modeling A-share microstructure: lot sizes,
the 2023-08-10 Main Board increment change, STAR 200-share minimum/odd-lot
rules, limit-up/down, suspensions, volume caps, costs, and slippage.

Two layers: research (continuous weights → return/Sharpe/turnover/Fitness,
no IC per repo convention) and execution (state-dependent lot rounding +
two-stage greedy exposure repair + next-open reference simulator).

Wires `portfolio build/simulate/eval` into the CLI and adds the
POSITION/FILL/PNL schema contracts. Covered by tests/test_portfolio.py.

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

pipeline/portfolio/__init__.py

@@ -0,0 +1,6 @@
+"""Portfolio construction phase.
+
+Turns continuous alpha/combo weights into tradable A-share positions under
+date-aware lot/board rules, and simulates execution with market constraints,
+costs, and slippage.
+"""