viewer: the overview's numbers checked against ledger-core

Kpis gets a spec (cash, liabilities, receivables, month-to-date against
the same days of last month, fiscal year to date, burn). tests/view_model.rs
computes each from ledger-core on random books (month ends, leap days,
any fiscal start) along with the period ranges, cash history, account
labels, journal count, receipt rule, origins and parties, the blob
root, and integrity counts. Burn rounds half away from zero, like the
rest of wanderbooks (it was banker's rounding).

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

crates/ledger-gui/src/books.rs

@@ -132,6 +132,30 @@ impl Integrity {
 }
 
 /// The headline numbers.
+///
+/// ```text
+/// GOAL: the overview's numbers are the books' numbers.
+/// SOURCE: the module's rule ("every number here comes from ledger-core;
+///         this module only indexes and arranges"); DECISION (the
+///         comparisons and the burn estimate, as defined here).
+/// INVARIANTS (all at `today`; m0 = the first of today's month):
+///   1. cash, cash_30d_ago: the cash accounts' balances (normal side) at
+///      the end of today and of today - 30 days.
+///   2. liabilities: the balance sheet's total liabilities at today.
+///   3. receivables: the balances of the asset accounts whose name says
+///      "receivable".
+///   4. revenue_mtd, expenses_mtd: the P&L from m0 through today.
+///      revenue_prev_month, expenses_prev_month: the P&L from the first
+///      of last month through the same day of it (its last day, if
+///      shorter).
+///   5. net_ytd: revenue minus expenses from the fiscal year's first day
+///      (ledger_config) through today.
+///   6. monthly_burn: over the three full months before m0, expenses
+///      minus revenue, divided by the months of those the books cover
+///      (from the first posted entry's month); Some only when positive,
+///      rounded to the cent half away from zero.
+///   7. first_entry: the earliest posted entry's date.
+/// ```
 #[derive(Clone, Debug, Default)]
 pub struct Kpis {
     pub cash: Decimal,
@@ -437,7 +461,10 @@ impl Books {
                 Some(_) => 3,
                 None => 0,
             };
-            (loss > Decimal::ZERO && months > 0).then(|| (loss / Decimal::from(months)).round_dp(2))
+            (loss > Decimal::ZERO && months > 0).then(|| {
+                (loss / Decimal::from(months))
+                    .round_dp_with_strategy(2, rust_decimal::RoundingStrategy::MidpointAwayFromZero)
+            })
         });
         Kpis {
             cash: cash[1],