fix(settlement): per-currency netting, balance guards, Decimal rates

Settlement correctness cluster from CODE-REVIEW-2026-06 (#2, #8, #13)
plus libra-#38:

- format_net_settlement_entry now enforces the same inline balance
  constraint as the fiat formatter (payment = receivable - payable
  + credit) and grows an optional credit leg. An unbalanced
  settlement raises instead of reaching the ledger.
- on_invoice_paid settles only what the payment covers: a partial
  payment clears that much receivable; excess (or a payment with
  nothing owed) becomes user credit. Previously the full prior
  balance was cleared against a smaller payment, shipping unbalanced
  postings. Settlement links are attached only when the payment
  clears the full open balance, and only for same-currency entries.
- get_unsettled_entries_bql returns each entry's real posting
  currency (was hardcoded "EUR") and exact Decimal amount strings
  (was float). /receivables/settle nets only entries denominated in
  the settlement currency.
- fiat_rate/btc_rate metadata computed via Decimal (new
  fiat_rate_metadata helper) instead of float division — cost-basis
  records no longer carry float drift.
- format_posting_at_average_cost omits the cost braces when
  cost_currency is unset ("SATS {}" is invalid Beancount).
- Underpay error payload serializes amounts as exact Decimal strings.
- validate_metadata catches decimal.InvalidOperation so bad
  fiat_amount input becomes ValidationError (libra-#38); flipped the
  tracking xfail.

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

tests/test_settlement_api.py

@@ -10,6 +10,7 @@ Underpay without explicit entry-picks returns 400 with diff details so
 the operator can either pay the exact net or specify `settled_entry_links`.
 """
 import importlib
+from decimal import Decimal
 from uuid import uuid4
 
 import pytest
@@ -268,10 +269,12 @@ async def test_underpay_without_explicit_links_returns_400(
     assert r.status_code == 400, f"expected 400, got {r.status_code}: {r.text}"
     payload = r.json().get("detail")
     assert isinstance(payload, dict), f"expected structured detail, got {payload!r}"
-    assert payload.get("cash_paid") == 30.0
-    assert payload.get("net_obligation") == 100.0
-    assert payload.get("receivable_total") == 100.0
-    assert payload.get("payable_total") == 0.0
+    # Amounts are exact Decimal strings (not floats) so the operator can
+    # act on them without precision loss.
+    assert Decimal(payload.get("cash_paid")) == Decimal("30.00")
+    assert Decimal(payload.get("net_obligation")) == Decimal("100.00")
+    assert Decimal(payload.get("receivable_total")) == Decimal("100.00")
+    assert Decimal(payload.get("payable_total")) == Decimal("0")
 
 
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