test(core): every surviving mutant killed or argued equivalent

Max-testing pass, step 8. The final mutation run left 112 survivors
(89% caught). Three agents triaged them by file, by the playbook's
table: spec the boundary then test it, a law rather than one more
pinned value, assert the real value, never weaken a test. Every expected
value is hand-computed.

- reports.rs (35): 28 killed, chiefly cash flow (inclusive start date,
  each line's sign and label, a loss on disposal moving to investing,
  financing designation), general ledger and members' equity date
  boundaries, and the balanced() checks (a real report nudged by a cent
  must stop balancing). 7 equivalent: four >/>= after an early return on
  zero; two in dead code (removed in this commit); one sign of a zero
  cleaned on output.
- ledger.rs, kernel.rs, journal.rs, money.rs (52, plus one new): 49
  killed, including every replay_* mutant at the core level (ids,
  accumulation, status, vendor tags, out-of-order refusal), FX rates,
  0%/100% deductible boundaries, a dividend sweep with no income, the
  legacy member rounding at close, a bill paid on its own date, a
  liability-account reconciliation, mixed-currency entries (via serde),
  ISO 4217 minor units, Display/Debug. 3 equivalent: zero refused before
  a </<= check; a period whose end is in a closed period refused earlier;
  columns(0).
- schedules, tax, analysis, payouts, assets, form1099 (25): 23 killed
  (schedule length 1-600, recurring end on its start, frequency parsing
  and stepping, cost of goods in tax net, a zero payout, disposal on the
  purchase date, quarter starts, labels); 2 equivalent (zero adds zero;
  days_held is provably >= 1).

Two small production cleanups the triage surfaced: kernel::columns(0)
returned a negative-zero credit that would print "-0" (regression
a_zero_movement_has_no_negative_zero_column, shown to fail before); and
cash_flow computed a book value it never used.

Spec lines added where a boundary wasn't specified (cash flow's
inclusive start and line signs, members' equity start date, payout with
no deposit, disposal on the purchase date, bill paid on its own date).
`just mutants` now runs core and store tests (the store's replay tests),
not the whole workspace, which rebuilt the viewer for every mutant.

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

crates/ledger-core/src/money.rs

@@ -171,6 +171,37 @@ mod tests {
         );
     }
 
+    /// ISO 4217 list one (docs/spec/sources/iso-4217.md): one code from
+    /// each exponent class, and the codes with no minor unit.
+    #[test]
+    fn minor_units_follow_iso_4217() {
+        let units = |c: &str| Currency::new(c).unwrap().minor_units();
+        assert_eq!(units("USD"), Some(2));
+        assert_eq!(units("JPY"), Some(0));
+        assert_eq!(units("BHD"), Some(3));
+        assert_eq!(units("KWD"), Some(3));
+        assert_eq!(units("CLF"), Some(4));
+        assert_eq!(units("UYW"), Some(4));
+        assert_eq!(units("XAU"), None);
+        assert_eq!(units("XDR"), None);
+    }
+
+    #[test]
+    fn currency_and_money_print_their_code_and_amount() {
+        assert_eq!(Currency::new("usd").unwrap().to_string(), "USD");
+        assert_eq!(format!("{:?}", Currency::EUR), "Currency(EUR)");
+        assert_eq!(usd(d(1250)).to_string(), "12.50 USD");
+        assert_eq!(format!("{:?}", usd(d(-5))), "-0.05 USD");
+    }
+
+    #[test]
+    fn is_zero_means_exactly_zero() {
+        assert!(Money::zero(Currency::USD).is_zero());
+        assert!(usd(d(0)).is_zero());
+        assert!(!usd(d(1)).is_zero());
+        assert!(!usd(d(-1)).is_zero());
+    }
+
     #[test]
     fn checked_arithmetic_rejects_currency_mismatch() {
         let eur = Money::new(d(100), Currency::EUR);