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/analysis.rs

@@ -6,7 +6,8 @@
 
 use chrono::NaiveDate;
 use ledger_core::analysis::{
-    Interval, balance_history, balance_series, flows_by_interval, pnl_by_interval,
+    Interval, accounts_of_type, balance_history, balance_series, flows_by_interval, interval_start,
+    pnl_by_interval,
 };
 use ledger_core::{AccountType, Currency, Ledger, Money, Side};
 use rust_decimal::Decimal;
@@ -79,3 +80,36 @@ fn balances_pnl_and_flows_over_time() {
     let io: Vec<(Decimal, Decimal)> = both.iter().map(|f| (f.2, f.3)).collect();
     assert_eq!(io, [(dec(5000), dec(300)), (dec(1200), dec(0))]);
 }
+
+#[test]
+fn intervals_start_on_calendar_boundaries() {
+    for (day, want) in [
+        (d(2, 14), d(1, 1)),
+        (d(5, 31), d(4, 1)),
+        (d(11, 30), d(10, 1)),
+    ] {
+        assert_eq!(interval_start(day, Interval::Quarter), want, "{day}");
+        assert_eq!(interval_start(day, Interval::Year), d(1, 1), "{day}");
+    }
+    assert_eq!(interval_start(d(11, 30), Interval::Month), d(11, 1));
+}
+
+#[test]
+fn accounts_of_type_picks_from_the_chart_in_order() {
+    let mut l = Ledger::new(Currency::USD);
+    let mut a = |c: &str, t| l.add_account(c, c, t, None, false).unwrap();
+    let cash = a("1000", AccountType::Asset);
+    let card = a("2100", AccountType::Liability);
+    let rent = a("6600", AccountType::Expense);
+    let bank = a("1010", AccountType::Asset);
+    let assets = accounts_of_type(&l, &[AccountType::Asset]);
+    assert_eq!(assets.len(), 2);
+    assert!(assets.contains(&cash) && assets.contains(&bank));
+    let mut cash_like = accounts_of_type(&l, &[AccountType::Asset, AccountType::Liability]);
+    cash_like.sort();
+    let mut want = vec![cash, card, bank];
+    want.sort();
+    assert_eq!(cash_like, want);
+    assert!(!cash_like.contains(&rent));
+    assert!(accounts_of_type(&l, &[AccountType::Revenue]).is_empty());
+}