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/tests/payouts.rs

@@ -153,3 +153,33 @@ fn accounts_must_fit_their_roles() {
     p.revenue.clear();
     assert!(invalid(p), "needs revenue");
 }
+
+#[test]
+fn a_zero_payout_has_no_bank_line() {
+    let b = books();
+    // 10.00 of sales, 3.00 of fees, a 7.00 refund: nothing deposited.
+    let p = Payout {
+        deposit_account: b.bank,
+        deposit: dec("0.00"),
+        revenue: vec![line(b.sales, "10.00")],
+        fees: vec![line(b.fees, "3.00")],
+        adjustments: vec![PayoutAdjustment {
+            account: b.refunds,
+            side: Side::Debit,
+            amount: dec("7.00"),
+            class: None,
+        }],
+    };
+    let (plan, totals) = payout_plan(&p, b.l.chart()).unwrap();
+    assert_eq!(totals.deposit, Decimal::ZERO);
+    assert_eq!(
+        plan.iter()
+            .map(|(a, s, m, _)| (*a, *s, *m))
+            .collect::<Vec<_>>(),
+        [
+            (b.fees, Side::Debit, dec("3.00")),
+            (b.refunds, Side::Debit, dec("7.00")),
+            (b.sales, Side::Credit, dec("10.00")),
+        ]
+    );
+}