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).

test/spreadsheet-preview.browser.test.ts

@@ -15,6 +15,8 @@ async function workbookBytes(): Promise<Buffer> {
   const summary = workbook.addWorksheet('Summary');
   summary.getCell('A1').value = 'Local workbook';
   summary.getCell('B2').value = 42;
+  summary.getColumn(2).width = 18;
+  summary.getRow(2).height = 30;
   summary.mergeCells('A3:C3');
   summary.getCell('A3').value = 'Merged cells';
   summary.getCell('A100').value = 'Far row';
@@ -92,6 +94,24 @@ describe('spreadsheet preview browser boundary', () => {
     });
     expect(initialGeometry.left).toBeGreaterThanOrEqual(36);
     expect(initialGeometry.top).toBeGreaterThanOrEqual(20);
+    // Headings line up with a custom-width column and a custom-height row.
+    const headingFit = await page.locator('.spreadsheet-grid').evaluate((grid) => {
+      const rect = (selector: string, text: string) => {
+        const element = [...grid.querySelectorAll(selector)].find((node) => node.textContent === text);
+        if (!element) throw new Error(`Missing ${selector} ${text}`);
+        return element.getBoundingClientRect();
+      };
+      const cell = rect('.spreadsheet-cell', '42');
+      const column = rect('.spreadsheet-column-heading', 'B');
+      const row = rect('.spreadsheet-row-heading', '2');
+      return {
+        cell: [cell.left, cell.width, cell.top, cell.height],
+        heading: [column.left, column.width, row.top, row.height],
+      };
+    });
+    expect(headingFit.cell[1]).toBe(126);
+    expect(headingFit.cell[3]).toBe(40);
+    expect(headingFit.heading).toEqual(headingFit.cell);
     await page.locator('.spreadsheet-grid').evaluate((grid) => {
       grid.scrollTop = 400;
       grid.scrollLeft = 400;