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/src/money.rs

@@ -39,6 +39,24 @@ impl Currency {
         Ok(Currency(upper))
     }
 
+    /// The currency's minor units: how many decimal places its smallest
+    /// unit has (2 for USD's cents, 0 for JPY, 3 for KWD), or `None` for
+    /// codes ISO 4217 gives none (gold, SDRs). Unlisted codes are 2.
+    ///
+    /// SOURCE: ISO 4217 list one, published 2026-09-17
+    /// (docs/spec/sources/iso-4217.md); generated from it, not typed in.
+    pub fn minor_units(&self) -> Option<u32> {
+        match self.code() {
+            "BIF" | "CLP" | "DJF" | "GNF" | "ISK" | "JPY" | "KMF" | "KRW" | "PYG" | "RWF"
+            | "UGX" | "UYI" | "VND" | "VUV" | "XAF" | "XOF" | "XPF" => Some(0),
+            "BHD" | "IQD" | "JOD" | "KWD" | "LYD" | "OMR" | "TND" => Some(3),
+            "CLF" | "UYW" => Some(4),
+            "XAG" | "XAU" | "XBA" | "XBB" | "XBC" | "XBD" | "XDR" | "XPD" | "XPT" | "XSU"
+            | "XTS" | "XUA" | "XXX" => None,
+            _ => Some(2),
+        }
+    }
+
     /// The three-letter code, e.g. `"USD"`.
     pub fn code(&self) -> &str {
         std::str::from_utf8(&self.0).expect("currency codes are ASCII by construction")