test(core): spec-blind suites and a live GnuCash oracle; four fixes they found

Max-testing pass, steps 2-4.

Spec-blind tests (rank 1-2): four agents, each given only the specs and
the public interface (doc comments and signatures, every body removed,
never src/ or the existing tests), wrote tests/spec_reports.rs (62),
spec_ledger.rs (72), spec_arap.rs (42), spec_assets_payroll.rs (50):
hand-computed examples per scenario and a proptest law per invariant
(conservation, partition, invariance, scaling, reversal cancels,
"a refusal leaves the books unchanged" over random invalid commands).
They found:

1. A refused payroll run changed the books: an all-zero run passed
   validation, then failed at post after creating seven payroll
   accounts and using up an entry id. validate_payroll_lines now
   refuses a run with no gross pay.
2. A refused add_account used up an account id (empty code, duplicate,
   unknown parent). The id is now taken only when the account is added.
3. Stale prose: close_period_to and members_equity still described the
   old last-member rounding.
4. Open (left #[ignore]d pending Stephen): depreciation_schedule's doc
   says "through the month containing" the date; the code prorates that
   month by days.

GnuCash oracle (rank 3, an oracle never the spec): `just oracle` builds
GnuCash 5.10 in a container pinned by base digest and package versions
(oracle/gnucash/) and runs tests/oracle_gnucash.rs: 200 seeded random
books built through wanderbooks (entries, foreign lines, reversals,
monthly closes), every account's balance at month ends and random dates
and each month's P&L, compared exactly: 15,800 comparisons. First run:
4,724 differences, all from foreign lines; without them, zero.
Adjudicated from principles (rules.md B-3): wanderbooks booked
foreign * rate unrounded ($108.34567); a ledger in one money holds no
fractions of its smallest unit, and its cash must match a bank. Stephen
decided: book in the base currency's minor unit, half away from zero.
add_line_fx now rounds; minor units per ISO 4217 list one (2026-09-17,
SIX), generated into Currency::minor_units and recorded in
docs/spec/sources/iso-4217.md. Replay now takes the journaled base
amount (replay_add_line_fx), so lines booked before replay exactly as
booked (contract: books_written_under_older_rules_still_open). Now:
zero differences, foreign lines included.

Tests: 449, from 223.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

oracle/gnucash/Dockerfile

@@ -0,0 +1,10 @@
+# GnuCash as a cross-check oracle (docs/oracle.md, "Who is right"): one
+# independent implementation of double entry we diff against. Its answers
+# are evidence, never the spec. Pinned so every run uses the same GnuCash.
+FROM debian@sha256:a29215f6a35e51e22adffa17f89e9d2ef06214e64a2bad10d765c46aea49f11f
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+      gnucash=1:5.10-0.1+b1 python3-gnucash=1:5.10-0.1+b1 \
+ && rm -rf /var/lib/apt/lists/*
+COPY books.py /oracle/books.py
+ENTRYPOINT ["python3", "/oracle/books.py"]

oracle/gnucash/books.py

@@ -0,0 +1,96 @@
+"""Build books in GnuCash and report its balances.
+
+stdin: a JSON list of books:
+  {"accounts": [{"id": 1, "code": "1000", "name": "Cash", "type": "Asset"}],
+   "entries":  [{"date": "2026-01-15", "description": "...",
+                 "lines": [{"account": 1, "side": "debit", "amount": "12.34"}]}],
+   "as_of":    ["2026-01-31", ...]}
+stdout: a JSON list, one per book:
+  {"balances": {"2026-01-31": {"1": "12.34", ...}, ...}}
+Balances are GnuCash's own: the sum of split values on the account through
+the end of that day, debits positive, as an exact decimal string.
+"""
+import datetime, json, os, sys, tempfile
+from decimal import Decimal
+import gnucash
+from gnucash import Account, GncNumeric, Session, SessionOpenMode, Split, Transaction
+
+TYPES = {
+    "Asset": gnucash.ACCT_TYPE_ASSET,
+    "Liability": gnucash.ACCT_TYPE_LIABILITY,
+    "Equity": gnucash.ACCT_TYPE_EQUITY,
+    "Revenue": gnucash.ACCT_TYPE_INCOME,
+    "Expense": gnucash.ACCT_TYPE_EXPENSE,
+}
+
+
+def numeric(amount: str) -> GncNumeric:
+    d = Decimal(amount)
+    sign, digits, exp = d.as_tuple()
+    denom = 10 ** max(0, -exp)
+    num = int(d * denom)
+    return GncNumeric(num, denom)
+
+
+def decimal_of(n) -> str:
+    return str(Decimal(n.num()) / Decimal(n.denom()))
+
+
+def end_of_day(date: str) -> int:
+    d = datetime.date.fromisoformat(date)
+    return int(datetime.datetime(d.year, d.month, d.day, 23, 59, 59,
+                                 tzinfo=datetime.timezone.utc).timestamp())
+
+
+def run(book_spec, path):
+    s = Session(f"xml://{path}", SessionOpenMode.SESSION_NEW_STORE)
+    try:
+        book = s.book
+        root = book.get_root_account()
+        usd = book.get_table().lookup("CURRENCY", "USD")
+        accounts = {}
+        for a in book_spec["accounts"]:
+            acct = Account(book)
+            acct.SetName(f'{a["code"]} {a["name"]}')
+            acct.SetCode(a["code"])
+            acct.SetType(TYPES[a["type"]])
+            acct.SetCommodity(usd)
+            root.append_child(acct)
+            accounts[a["id"]] = acct
+        for e in book_spec["entries"]:
+            d = datetime.date.fromisoformat(e["date"])
+            tx = Transaction(book)
+            tx.BeginEdit()
+            tx.SetCurrency(usd)
+            tx.SetDate(d.day, d.month, d.year)
+            tx.SetDescription(e.get("description", ""))
+            for line in e["lines"]:
+                value = numeric(line["amount"])
+                if line["side"] == "credit":
+                    value = value.neg()
+                sp = Split(book)
+                sp.SetParent(tx)
+                sp.SetAccount(accounts[line["account"]])
+                sp.SetValue(value)
+                sp.SetAmount(value)
+            tx.CommitEdit()
+        out = {}
+        for date in book_spec["as_of"]:
+            t = end_of_day(date)
+            out[date] = {str(i): decimal_of(a.GetBalanceAsOfDate(t)) for i, a in accounts.items()}
+        return {"balances": out}
+    finally:
+        s.end()
+
+
+def main():
+    books = json.load(sys.stdin)
+    results = []
+    with tempfile.TemporaryDirectory() as tmp:
+        for i, b in enumerate(books):
+            results.append(run(b, os.path.join(tmp, f"book{i}.gnucash")))
+    json.dump(results, sys.stdout)
+
+
+if __name__ == "__main__":
+    main()