Removes core balance calculation logic

Migrates balance calculation and inventory tracking to
Fava/Beancount, leveraging Fava's query API for all
accounting calculations. This simplifies the core module
and centralizes accounting logic in Fava.

core/__init__.py

@@ -4,8 +4,6 @@ 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:
@@ -13,16 +11,14 @@ Benefits:
 - Reusable across different storage backends
 - Clear separation of concerns
 - Easier to audit and verify
+
+Note: Balance calculation and inventory tracking have been migrated to Fava/Beancount.
+All accounting calculations are now performed via Fava's query API.
 """
 
-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",