tax summary: Schedule K items stated separately; XLSX never shows a wrong amount

From the IRS form instructions (now quoted in docs/spec/sources/
irs-tax-lines.md):
- On Forms 1065 and 1120-S, Schedule K takes "only taxable portfolio
  interest", and charitable contributions are among the items the entity
  "must state separately". The tax summary counted both in ordinary
  income and deductions; they now go in TaxSummary::separately_stated,
  outside book_net and tax_net, and the report, MCP export, and viewer
  list them under "Separately stated to owners (Schedule K)".
- Schedule C: "Do not include charitable contributions". The category no
  longer applies to Schedule C (a sole proprietor's gift is personal, an
  owner draw).
- interest_income's note: interest earned in the business itself is
  ordinary income (other_income).

XLSX: a cell number is a binary double and Excel honors 15 significant
digits (Microsoft, "Excel specifications and limits"), so an amount over
$9,999,999,999,999.99 or finer than a cent is written as its exact text,
right-aligned, instead of a number Excel would show wrong. The old
conversion also fell back to 0.0 silently; gone. Property test: every
whole-cent amount up to 15 digits round-trips exactly.

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

crates/ledger-import/src/report.rs

@@ -460,6 +460,14 @@ pub fn tax_summary(ts: &ledger_core::TaxSummary) -> Statement {
             diff(ts.book_net(), ts.tax_net()),
         ],
     ));
+    if !ts.separately_stated.is_empty() {
+        rows.push(spacer());
+        rows.push(section("Separately stated to owners (Schedule K)"));
+        rows.push(subheading(
+            "Portfolio interest and charitable contributions; not included above:",
+        ));
+        rows.extend(lines(&ts.separately_stated));
+    }
     if !ts.uncategorized.is_empty() {
         rows.push(spacer());
         rows.push(section("Not yet categorized"));