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

.gitignore

@@ -5,3 +5,4 @@ site/dist/
 .netlify/
 /mutants.out*/
 /mutants-store/
+/mutants-import/