fix(preview): address review of the XLSX preview

- Normalize the value shapes ExcelJS loads before formatting: Date cells are
  formatted from their serial (UTC), so they no longer render as a local-time
  string a day early at negative UTC offsets; rich text joins its runs,
  hyperlinks show their text, error values show the error, and formula and
  shared-formula results (including error results) recurse. Excel serials are
  rounded to whole milliseconds so 00:05 no longer shows as 00:04.
- sendTile() skips hidden rows and columns, and at the 2500-cell cap returns a
  truncated tile with a warning instead of failing the whole preview.
- ExcelJS now parses a STORE-only archive rebuilt from exactly the entries
  admitXlsx() inflated and counted, never the fetched bytes. Admission walks
  local headers while JSZip reads the central directory, so overlapping
  entries could show the two readers different sheets. A duplicate local
  entry name is refused. The theme fallback reads the admitted entry too.
- Row and column headings take their size from the same axis math as cells.
- Document the admission, worker-only loading and SPREADSHEET_ASSET_VERSION
  rules in architecture-invariants, and list .xlsx in the attachments panel
  help and the `codeman attach` error text (built from the accepted list).

docs/architecture-invariants.md

@@ -358,6 +358,8 @@ A file path an agent prints is a link on both surfaces it can appear on, and cli
 
 ⚠️ **The preview overlay must outrank the panel that launched it.** `.file-preview-overlay` sits at `z-index: 5100`, above the response viewer (5000) and its backdrop (4999); at its historical 2000 a path clicked in the chat opened the overlay *behind* the chat, which reads as a dead link. It stays below the toast/picker band (10000+) so a "Saved" toast still lands on top.
 
+**XLSX previews parse in the browser worker, and ExcelJS only ever sees what admission checked.** An `.xlsx` joins the raw routes like any other file (`?preview=true` caps it at 10 MB with a 413); the server never parses it. `spreadsheet-preview.js` hands the bytes to `spreadsheet-preview-worker.js`, the ONLY place the pinned `exceljs`/`fflate` vendor bundles load: fflate and `spreadsheet-xlsx-core.js` at worker start, ExcelJS only after `admitXlsx()` passes, and the page itself never loads either. `admitXlsx()` streams every entry through fflate and enforces the entry count, per-entry and total inflated bytes (64 MB), compression ratio, worksheet, cell, merge and style caps on the actual inflated output, refusing (never truncating) a workbook that trips one. ⚠️ Admission walks LOCAL headers while ExcelJS (JSZip) reads the CENTRAL directory, so overlapping entries (a stored entry hiding a whole `sheet1.xml`, a one-cell decoy later in the stream) once let a file be admitted as 1 cell and parsed as 300k. The worker therefore never hands ExcelJS the fetched bytes: it gets `buildAdmittedArchive()`, a STORE-only `fflate.zipSync(entries, { level: 0 })` of exactly the entries admission inflated, and a name streamed twice is refused. That costs one transient copy of the inflated entries (bounded by the same 64 MB cap) and is pinned by the overlapping-entry fixture in `test/spreadsheet-preview-worker.test.ts`. The worker is a stable URL cache-busted by `SPREADSHEET_ASSET_VERSION` in `spreadsheet-preview.js`, the content hash of the worker, the core and both vendor bundles; `npm run check:public-assets` fails when it drifts, so editing any of those (or bumping either package) means updating the token, or a deploy pairs a new worker with a year-cached old core.
+
 ### Filesystem path picker
 
 **Filesystem path picker** (Link Existing "Browse" button + the extended mobile keyboard's `📁 Path` key): a lazy one-directory-at-a-time browser over `GET /api/filesystem/browse`, with `GET /api/filesystem/preview` serving the tapped file. It starts at the active session's working directory (falling back to the Codeman Cases root, then `/mnt/d`, then the first root), hides dot entries, and inserts the chosen path **without** Enter so the prompt is not submitted. The companion `⌫ All` key clears only the current unsent prompt buffer and must never emit the agent's `/clear` command.