feat(fusion_accounting_bank_rec): exchange_diff helper for FX gain/loss pre-check

Made-with: Cursor

fusion_accounting_bank_rec/services/exchange_diff.py

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+"""Exchange-difference calculation helper.
+
+Pure-Python FX gain/loss computation. The engine uses this for rapid
+pre-checks; Odoo's account.move._create_exchange_difference_move() is
+invoked separately for the actual GL posting.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass
+class ExchangeDiffResult:
+    needs_diff_move: bool
+    diff_amount: float           # in company currency; positive = gain, negative = loss
+    line_company_amount: float
+    against_company_amount: float
+
+
+def compute_exchange_diff(*, line_amount, line_currency_code, against_amount,
+                            against_currency_code, line_rate, against_rate) -> ExchangeDiffResult:
+    """Compute whether an exchange-diff move is needed and its magnitude.
+
+    Args:
+        line_amount: Bank line amount in its currency
+        line_currency_code: e.g. 'USD'
+        against_amount: Matched journal item amount in its currency
+        against_currency_code: e.g. 'USD' (or different)
+        line_rate: FX rate (foreign per company currency) at line date
+        against_rate: FX rate at journal item posting date
+
+    Returns:
+        ExchangeDiffResult with needs_diff_move flag and computed diff
+        in company currency (positive = gain, negative = loss).
+    """
+    line_company = line_amount * line_rate
+    against_company = against_amount * against_rate
+
+    diff = line_company - against_company
+    needs_diff = abs(diff) > 0.005  # rounding tolerance
+
+    return ExchangeDiffResult(
+        needs_diff_move=needs_diff,
+        diff_amount=round(diff, 2),
+        line_company_amount=round(line_company, 2),
+        against_company_amount=round(against_company, 2),
+    )