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

crates/ledger-core/tests/invariants.rs

@@ -1016,3 +1016,49 @@ fn a_line_on_an_unknown_or_placeholder_account_is_refused_when_added() {
         "refused lines aren't added"
     );
 }
+
+/// Spec (rules.md B-3; DECISION, Stephen 2026-10-13, after the GnuCash
+/// cross-check): a foreign line is booked in the base currency's smallest
+/// unit, half away from zero; the foreign original is kept.
+#[test]
+fn a_foreign_line_is_booked_in_whole_base_units() {
+    let book = |base: Currency, foreign: Currency, amount: &str, rate: &str| {
+        let mut l = Ledger::new(base);
+        let a = l
+            .add_account("1000", "Cash", AccountType::Asset, None, false)
+            .unwrap();
+        let e = l.create_entry(date(5), "fx");
+        let r = l.add_line_fx(
+            e,
+            a,
+            Side::Debit,
+            foreign,
+            amount.parse().unwrap(),
+            rate.parse().unwrap(),
+        );
+        r.map(|()| {
+            let line = l.get_entry(e).unwrap().lines()[0].clone();
+            (line.amount.amount, line.foreign_amount)
+        })
+    };
+    let dec = |s: &str| s.parse::<Decimal>().unwrap();
+    // 100.00 EUR x 1.0834567 = 108.34567 USD -> 108.35.
+    assert_eq!(
+        book(Currency::USD, Currency::EUR, "100.00", "1.0834567").unwrap(),
+        (dec("108.35"), Some(dec("100.00")))
+    );
+    // 10.00 USD x 151.237 = 1512.37 JPY -> 1512 (yen have no minor unit).
+    assert_eq!(
+        book(Currency::JPY, Currency::USD, "10.00", "151.237")
+            .unwrap()
+            .0,
+        dec("1512")
+    );
+    // Exactly half a cent rounds away from zero.
+    assert_eq!(
+        book(Currency::USD, Currency::EUR, "1", "0.125").unwrap().0,
+        dec("0.13")
+    );
+    // Rounds to nothing: refused.
+    assert!(book(Currency::USD, Currency::EUR, "0.001", "1").is_err());
+}