feat(portfolio): add stock consolidation domain and schema

app/domain/corporate_actions.py

@@ -0,0 +1,37 @@
+"""Pure domain rules for portfolio stock consolidations."""
+from __future__ import annotations
+from dataclasses import dataclass
+from decimal import Decimal, ROUND_HALF_UP
+
+class CorporateActionError(ValueError):
+    code = "INVALID_CORPORATE_ACTION_RATIO"
+    def __init__(self, message: str, code: str | None = None):
+        super().__init__(message); self.code = code or self.code
+
+@dataclass(frozen=True)
+class StockConsolidation:
+    quantity_before: Decimal
+    quantity_after: Decimal
+    average_price_before: Decimal
+    average_price_after: Decimal
+    total_cost_basis: Decimal
+    ratio: Decimal
+
+def calculate_stock_consolidation(quantity_before: Decimal, quantity_after: Decimal, average_price_before: Decimal) -> StockConsolidation:
+    q0, q1, wac = map(Decimal, (quantity_before, quantity_after, average_price_before))
+    if q0 <= 0 or q1 <= 0 or wac < 0:
+        raise CorporateActionError("quantities must be positive and price non-negative")
+    ratio = q1 / q0
+    if ratio >= 1 or ratio <= 0:
+        raise CorporateActionError("stock consolidation must reduce quantity")
+    if q1 != q1.to_integral_value() or q0 != q0.to_integral_value():
+        raise CorporateActionError("fractional result unsupported", "FRACTIONAL_RESULT_UNSUPPORTED")
+    total = (q0 * wac).quantize(Decimal("0.00000001"), rounding=ROUND_HALF_UP)
+    after = (total / q1).quantize(Decimal("0.00000001"), rounding=ROUND_HALF_UP)
+    if after * q1 != total:
+        raise CorporateActionError("fractional result unsupported", "FRACTIONAL_RESULT_UNSUPPORTED")
+    return StockConsolidation(q0, q1, wac, after, total, ratio)
+
+def apply_corporate_action_to_wac(quantity: Decimal, wac: Decimal, quantity_after: Decimal) -> tuple[Decimal, Decimal]:
+    result = calculate_stock_consolidation(quantity, quantity_after, wac)
+    return result.quantity_after, result.average_price_after