Completes core logic refactoring (Phase 3)

Refactors the accounting logic into a clean, testable core module, separating business logic from database operations.

This improves code quality, maintainability, and testability by creating a dedicated `core/` module, implementing `CastleInventory` for position tracking, moving balance calculations to `core/balance.py`, and adding comprehensive validation in `core/validation.py`.

core/__init__.py

@@ -0,0 +1,29 @@
+"""
+Castle Core Module - Pure accounting logic separated from database operations.
+
+This module contains the core business logic for double-entry accounting,
+following Beancount patterns for clean architecture:
+
+- inventory.py: Position tracking across currencies
+- balance.py: Balance calculation logic
+- validation.py: Comprehensive validation rules
+
+Benefits:
+- Testable without database
+- Reusable across different storage backends
+- Clear separation of concerns
+- Easier to audit and verify
+"""
+
+from .inventory import CastleInventory, CastlePosition
+from .balance import BalanceCalculator
+from .validation import ValidationError, validate_journal_entry, validate_balance
+
+__all__ = [
+    "CastleInventory",
+    "CastlePosition",
+    "BalanceCalculator",
+    "ValidationError",
+    "validate_journal_entry",
+    "validate_balance",
+]