fix(portfolio): tolerate consolidation WAC residue

app/domain/corporate_actions.py

@@ -6,6 +6,13 @@ import json
 from dataclasses import dataclass
 from decimal import ROUND_HALF_UP, Decimal
 
+STORAGE_QUANTUM = Decimal("0.00000001")
+
+
+def quantize_stored_decimal(value: Decimal) -> Decimal:
+    """Round a monetary value to the NUMERIC(*, 8) persistence contract."""
+    return value.quantize(STORAGE_QUANTUM, rounding=ROUND_HALF_UP)
+
 
 def canonical_payload_hash(payload: object) -> str:
     if hasattr(payload, "model_dump"):
@@ -36,11 +43,13 @@ def calculate_stock_consolidation(quantity_before: Decimal, quantity_after: Deci
         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)
+    # Preserve the authoritative pre-action total independently. A per-unit WAC
+    # rounded to the storage scale cannot always multiply back to that total
+    # exactly, and that harmless decimal residue is not a fractional share.
+    total = quantize_stored_decimal(q0 * wac)
+    before = quantize_stored_decimal(wac)
+    after = quantize_stored_decimal(total / q1)
+    return StockConsolidation(q0, q1, before, 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)