fix(assertions): send Balance directives in Fava's JSON shape (libra-#39)

format_balance returned a Beancount source string, but fava.add_entry
feeds PUT /add_entries whose deserialiser expects
{"t": "Balance", "amount": {"number", "currency"}, ...} — every
assertion create 500'd. Returns the dict shape now.

The seven strict-xfail reconciliation tests tracking this flip to
regular passing tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

beancount_format.py

@@ -115,13 +115,18 @@ def format_balance(
     account: str,
     amount: int,
     currency: str = "SATS"
-) -> str:
+) -> Dict[str, Any]:
     """
-    Format a balance assertion directive for Beancount.
+    Format a balance assertion directive for Fava's JSON API.
 
     Balance assertions verify that an account has an expected balance on a specific date.
     They are checked automatically by Beancount when the file is loaded.
 
+    Fava's `deserialise` (fava/serialisation.py) expects
+    `{"t": "Balance", "amount": {"number", "currency"}, ...}` — the
+    previous source-string return 500'd on every assertion create
+    (libra-#39).
+
     Args:
         date_val: Date of the balance assertion
         account: Account name (e.g., "Assets:Bitcoin:Lightning")
@@ -129,15 +134,15 @@ def format_balance(
         currency: Currency code (default: "SATS")
 
     Returns:
-        Beancount balance directive as a string
-
-    Example:
-        >>> format_balance(date(2025, 11, 10), "Assets:Bitcoin:Lightning", 1500000, "SATS")
-        '2025-11-10 balance Assets:Bitcoin:Lightning  1500000 SATS'
+        Fava API Balance entry dict ready for `fava.add_entry`.
     """
-    date_str = date_val.strftime('%Y-%m-%d')
-    # Two spaces between account and amount (Beancount convention)
-    return f"{date_str} balance {account}  {amount} {currency}"
+    return {
+        "t": "Balance",
+        "date": date_val.strftime('%Y-%m-%d'),
+        "account": account,
+        "amount": {"number": str(amount), "currency": currency},
+        "meta": {},
+    }
 
 
 def format_posting_with_cost(