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-import/src/templates.rs

@@ -307,6 +307,12 @@ pub fn chart_template(entity: EntityType, owners: &[String]) -> Result<ChartTemp
             "expense",
             Some("nondeductible"),
         ),
+        acct(
+            "6995",
+            "Purchase Discounts",
+            "expense",
+            Some("other_deductions"),
+        ),
         acct(
             "7100",
             "Employee Benefits",