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

docs/testing.md

@@ -29,6 +29,7 @@ next (gap list).
 | Benchmarks | a baseline (6) | `ledger-core/benches/books.rs`, `ledger-store/benches/store.rs` | `just bench` | ~2 min | none (no budget, by decision) |
 | Mutation | can the tests fail? | `ledger-core` (2026-10-13: 1,011 caught + 13 timeouts of 1,024 viable, 0 missed) | `just mutants` | ~3 h | weekly |
 | Mutation (store) | can the tests fail? | `ledger-store` (2026-10-13: 357 caught + 1 timeout of 365 viable, from 82%; three of its seven survivors then killed; four equivalent left: `sort_value`'s two arms, `OpenFlags` `|` vs `^`, the long form of a sweepless close) | `just mutants-store` | ~12 min | weekly |
+| Mutation (import) | can the tests fail? | `ledger-import` (2026-10-13: from 91 of 171 (53%) to 208 of 239 viable on the full run; the 31 survivors (XLSX layout, borders, alignment, exact-text grouping, IIF transliteration, tax-summary optional sections, day-equals-month dates) then killed by targeted tests, re-run clean but one: a blank amount cell written unbordered on an unruled row, invisible, so equivalent) | `just mutants-import` | ~1 h | weekly |
 
 CI: `.github/workflows/check.yml` runs `just check` on every push;
 `scheduled.yml` runs proofs, the oracle, fuzzing, and mutation weekly.
@@ -111,5 +112,5 @@ B-14) are the first.
 | Lean: apportion's sum law for any n | OPEN | Kani covers n <= 3, properties n <= 5; needs a permutation lemma |
 | ASC paragraphs not yet read | OPEN | ASC 360-10-35-4, 340-10, 230-10-45-10, 606-10-55-37B, 210: needs a FASB basic-view login |
 | GnuCash's own close | OPEN | the oracle books closes as ordinary entries; comparing GnuCash's Close Book needs its Scheme API |
-| Import: spec-blind suite, footing properties, mutation | IN PROGRESS | sources vendored (Intuit bank CSV and IIF, ECMA-376/Excel numbers, IRS form lines); fuzz_import_csv; templates checked against the engine's control accounts |
+| Import: spec-blind suite, footing properties, mutation | DONE | sources vendored (Intuit bank CSV and IIF, ECMA-376/Excel numbers, IRS form lines); specs on every module; tests/spec_import.rs (spec-blind, footing and round-trip properties, XLSX read back from its XML, layout and styles); fuzz_import_csv; templates checked against the engine's control accounts; mutation one equivalent left |
 | MCP, config, GUI crates | NEXT | the same playbook |