import: XLSX layout spec and tests; mutation 53% -> one equivalent left

report_to_xlsx gains INVARIANTS 4-6 (header block, right-aligned amount
headings, spacer and account spacing, footer one empty row down, rules
exactly where rule_above says, exact text grouped in threes), tested by
reading the workbook's styles, merges and column widths. The general
ledger's footer now sits one empty row down like the other sheets.

Also killed: IIF transliteration arms, tax summary's optional sections,
and day-equals-month bank dates. `just mutants-import`, weekly in CI.

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

crates/ledger-import/src/xlsx.rs

@@ -175,6 +175,15 @@ fn number(
 ///      text, right-aligned. NEVER a number Excel would show wrong.
 ///   2. Every statement row is a row, in order, with its label.
 ///   3. The output is a valid XLSX (a zip holding a workbook).
+///   4. Layout: the company, title, and period on rows 1-3, each merged
+///      across the sheet's columns and centered; column headings on row 5,
+///      those over amounts right-aligned; content from row 6. A spacer is
+///      an empty row, the general ledger's accounts are one empty row
+///      apart, and the footer is one empty row below the last content.
+///   5. A statement row with `rule_above` (crate::report) carries a thin
+///      top border across every amount column, blank ones included; no
+///      other row's amounts do.
+///   6. Exact text groups thousands in threes ("1,234,567.125").
 /// ```
 pub fn report_to_xlsx(report: &XlsxReport<'_>, meta: &ReportMeta) -> Result<Vec<u8>, XlsxError> {
     let mut workbook = Workbook::new();
@@ -262,7 +271,10 @@ fn general_ledger(
         sheet.write_string_with_format(row, i as u16, h.to_uppercase(), fmt)?;
     }
     row += 1;
-    for a in &gl.accounts {
+    for (n, a) in gl.accounts.iter().enumerate() {
+        if n > 0 {
+            row += 1; // an empty row between accounts
+        }
         let name = format!("{} {}", a.code, a.name);
         sheet.write_string_with_format(row, 0, &name, &f.bold)?;
         row += 1;
@@ -290,7 +302,7 @@ fn general_ledger(
         number(sheet, row, 4, a.total_debits, &f.dollars_sum)?;
         number(sheet, row, 5, a.total_credits, &f.dollars_sum)?;
         number(sheet, row, 6, a.ending_balance, &f.dollars_sum)?;
-        row += 2;
+        row += 1;
     }
     for (c, w) in [
         (0, 12.0),