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/tax.rs

@@ -228,3 +228,39 @@ fn forms_parse_from_common_spellings() {
     }
     assert_eq!(TaxForm::parse("990"), None);
 }
+
+#[test]
+fn cost_of_goods_reduces_net_like_a_deduction() {
+    let mut l = books();
+    let cash = l.chart().find_by_code("1000").unwrap().id;
+    let cogs = l
+        .add_account("5000", "Cost of sales", AccountType::Expense, None, false)
+        .unwrap();
+    l.set_account_tax(cogs, Some("cost_of_goods_sold"), None)
+        .unwrap();
+    let e = l.create_entry(d(2, 1), "x");
+    let m = Money::new(dec("1000"), Currency::USD);
+    l.add_line(e, cogs, Side::Debit, m).unwrap();
+    l.add_line(e, cash, Side::Credit, m).unwrap();
+    l.post_entry(e).unwrap();
+    let ts = tax_summary(&l, d(1, 1), d(3, 31), Some(TaxForm::Form1065)).unwrap();
+    assert_eq!(ts.cost_of_goods.len(), 1);
+    assert_eq!(ts.cost_of_goods[0].book, dec("1000"));
+    assert_eq!(ts.cost_of_goods[0].tax, dec("1000"));
+    // The fixture's nets, less 1,000 of cost of goods sold.
+    assert_eq!(ts.book_net(), dec("4276.67"));
+    assert_eq!(ts.tax_net(), dec("4483.33"));
+}
+
+#[test]
+fn forms_have_their_irs_names() {
+    let labels: Vec<&str> = TaxForm::ALL.iter().map(|f| f.label()).collect();
+    assert_eq!(
+        labels,
+        ["Schedule C", "Form 1065", "Form 1120-S", "Form 1120"]
+    );
+    // A label parses back to its form.
+    for f in TaxForm::ALL {
+        assert_eq!(TaxForm::parse(f.label()), Some(f));
+    }
+}