fix(preview): normalize entry names the way ExcelJS sees them, skip defined names, pin fflate 0.8.3

Admission checked ZIP entry names as stored, but JSZip (inside ExcelJS)
resolves `.`, `..` and empty segments on load, and ExcelJS strips one
leading `/` and matches worksheets with an unanchored pattern. Names like
`/xl/worksheets/sheet1.xml` or `xl/worksheets/sheet1.xml.x` skipped every
counter. Admission now computes the name ExcelJS will see for each entry,
refuses two entries that resolve to the same name, keys the rebuilt
archive on it, and picks the worksheet/styles counters from it.

ExcelJS's DefinedNames model setter expands every range into one object
per cell. The preview never shows defined names, so the worker stubs
`_definedNames.model` before load.

Pin fflate to 0.8.3 (GHSA-px8p-9vwx-vf98) and refresh
SPREADSHEET_ASSET_VERSION.

docs/architecture-invariants.md

@@ -362,7 +362,7 @@ 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`. ⚠️ 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 `<mergeCell>`/`<col>` attributes IN ORDER from the tag name with a sticky regex that consumes each quoted value whole (a raw `>` or the other quote character is legal inside a value, so a first-match search could be fed a fake `ref`/`max`), and refuses a tag whose attributes do not parse up to `>` or repeat a name. It counts every `<row>` (ExcelJS keeps one Row object per element, cells or not) against per-sheet and total row caps, and every `<xf>` in styles.xml with no list-tracking state (a `</cellXfs>` inside a comment used to desync it). It carries everything from the last `<` into the next inflate chunk, since `<` can never appear inside an attribute value, 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.
+**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). Defined names expand the same way and live in `xl/workbook.xml`, which admission never scans: ExcelJS's `DefinedNames` model setter builds one object per cell of every range (a whole-sheet name exhausted a 4 GB heap), so right after `new ExcelJS.Workbook()` the worker replaces `_definedNames.model` with an `Object.defineProperty` stub whose getter returns `[]` and whose setter drops the value. Print areas and titles are split off in the workbook xform's reconcile, before that setter runs, so they are unaffected, and `defineProperty` throws if a future ExcelJS renames `_definedNames` rather than silently expanding again. ⚠️ Every name admission uses is the one ExcelJS will SEE, not the stored one: JSZip resolves each entry name on load (`utils.resolve`: `.` and empty middle segments dropped, `..` pops a segment) and ExcelJS then strips one leading `/` and matches worksheets with an UNANCHORED `xl/worksheets/sheet<N>.xml`. Checking the stored name once let `/xl/worksheets/sheet1.xml`, `xl/./...`, `xl//...`, `xl/xl/worksheets/sheet1.xml` and `xl/worksheets/sheet1.xml.x` skip every counter. `excelJsEntryName()` mirrors both steps; admission refuses two entries that resolve to one name, keys `inflatedEntries` (so the rebuilt archive) on the resolved name, picks the worksheet and styles counters from it, and counts any name matching ExcelJS's unanchored worksheet pattern as a worksheet. The local-versus-central consistency checks still compare the stored names. The counter reads `<mergeCell>`/`<col>` attributes IN ORDER from the tag name with a sticky regex that consumes each quoted value whole (a raw `>` or the other quote character is legal inside a value, so a first-match search could be fed a fake `ref`/`max`), and refuses a tag whose attributes do not parse up to `>` or repeat a name. It counts every `<row>` (ExcelJS keeps one Row object per element, cells or not) against per-sheet and total row caps, and every `<xf>` in styles.xml with no list-tracking state (a `</cellXfs>` inside a comment used to desync it). It carries everything from the last `<` into the next inflate chunk, since `<` can never appear inside an attribute value, 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