feat: auto-reconcile stale holding mirrors

Closes #94

trex_core/application/auto_reconcile.py

@@ -0,0 +1,46 @@
+"""Fail-closed holding mirror auto-reconciliation contracts (issue #94)."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import datetime
+from typing import Protocol
+from uuid import UUID
+
+
+@dataclass(frozen=True)
+class HoldingsCostBasisEvidence:
+    """Canonical portfolio-hub evidence captured by one successful HTTP read."""
+
+    portfolio_id: str
+    symbol: str
+    current_quantity: float
+    cost_basis_per_unit: float
+    total_cost_basis: float
+    observed_at: datetime
+    data_completeness_warning: str | None = None
+
+
+@dataclass(frozen=True)
+class AutoReconcileCommand:
+    """CAS-bound command for one atomic mirror repair."""
+
+    slot_id: UUID
+    instance_key: str
+    portfolio_id: str
+    symbol: str
+    expected_qty: float
+    expected_avg_price: float | None
+    target_qty: float
+    target_avg_price: float
+    hub_quantity: float
+    broker_quantity: int
+    evidence_observed_at: datetime
+
+
+class AutoReconcileStore(Protocol):
+    """Atomic qty/avg/audit/B-warning persistence boundary."""
+
+    def apply_holding_mirror(self, command: AutoReconcileCommand) -> bool:
+        """Apply exactly once when every CAS/eligibility condition still holds."""
+        ...