feat(package): aging CSVs in the export and the CPA package (P4b S6)

The CPA's first tie-out is that the aging equals the control account, so
books.export and the CPA package now write ar_aging.csv and ap_aging.csv
from one builder: one row per open document (number, party, dates, terms,
total, paid, discounted, outstanding, bucket, days overdue) and a TOTAL
line carrying the outstanding sum, the GL account and balance, and the
difference (always 0.00; B-23, B-38 INVARIANT 1). XLSX and PDF lay the
same aging out as a statement with the tie-out beneath it.

The package README states the tie-out, the gross discount method (A-8)
and the direct write-off method (A-5). The chart templates gain 6995
Purchase Discounts so no template puts another account at a control
role's code (4100 and 6060 already match). The viewer's Money out list
gains the discount-deadline column and colours a due-soon or
discount-open date; the screenshot is re-rendered.

Test: the_aging_csvs_tie_to_the_control_check in ledger-package; the
existing package file-order test and mcp_journeys see the two new files.

crates/ledger-gui/tests/invoices.rs

@@ -156,6 +156,8 @@ fn the_list_shows_the_ledgers_figures() {
     );
     let mut h = open(&dir, Route::Payables);
     shot(&mut h, "payables");
+    // The Money out list carries the discount-deadline column (blank
+    // until terms are journaled, S4.3).
     assert!(
         h.query_by_label_contains("Bill 1, Clay Co, Open, owes 300.00")
             .is_some()