fix(preview): bound what ExcelJS expands during XLSX admission

ExcelJS 4.4.0 expands three constructs into one object per cell or column
at load time, so a few KB admitted as one cell could cost a gigabyte:

- a <mergeCell> now costs its full area against the per-sheet and total
  cell caps, and a ref that does not parse is refused
- a <col> whose min or max is past 16384 is refused
- the worker loads with ignoreNodes: ['dataValidations']; the preview
  never shows validations, and a whole-column dropdown took 5 s

The XML counter now scans up to the last complete tag and carries the
rest, so a merge or col tag cut by an inflate-chunk edge is read whole.
A central-directory compressedSize that runs past the file is refused,
since the ratio cap divides by it.

The renderer and core axis offsets use prefix sums with a binary search
instead of walking every override per call.

docs/architecture-invariants.md

@@ -358,7 +358,7 @@ 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.
+**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`. ⚠️ Admission also has to bound what ExcelJS EXPANDS, not just what it reads: ExcelJS 4.4.0 builds one object per covered cell of a `<mergeCell>`, per address of a `<dataValidation sqref>` and per index up to `<col max>`, so a 6.5 KB file admitted as one cell once cost a gigabyte. The counter therefore charges each merge its full AREA against the cell caps (an unparseable `ref` is refused), refuses a `<col>` whose `min`/`max` is past 16384, and the worker loads with `ignoreNodes: ['dataValidations']` (the preview never shows validations). The counter reads tags whole across inflate-chunk edges (it scans only up to the last complete tag and carries the rest), and a central-directory `compressedSize` that runs past the file is refused, since the ratio cap divides by it. Fixtures for each are in `test/spreadsheet-preview-worker.test.ts` and `test/spreadsheet-xlsx-core.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