Merge #502: read-only XLSX spreadsheet preview in the file-preview overlay

# Conflicts:
#	CLAUDE.md

docs/architecture-invariants.md

@@ -379,6 +379,8 @@ 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`. ⚠️ 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`, where admission reads only the `<sheet>` ids: 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 reads each `<row>`'s attributes the same way. ⚠️ The INDEX a row or sheet claims is a cost too, not just how many there are: ExcelJS stores a row at `_rows[r - 1]` and walks `_rows` up to the largest index on every `eachRow` and `sheet.model` (five rows plus one `<row r="50000000">` took 5 s to load and 1.7 s per tile), and stores a sheet at `_worksheets[sheetId]`, which its `worksheets` getter slices and sorts (`sheetId="30000000"` on a one-cell workbook took 1.6 s and 557 MB). So a `<row r>` that is not plain digits in 1 to 1,048,576 is refused (an absent `r` is fine), and a counter for the resolved `xl/workbook.xml` reads every `<sheet>` tag in order and refuses one that does not parse or whose `sheetId` is not plain digits up to `LIMITS.maxSheetId` (65535; real ids are small, and the `(?=[\s/>])` lookahead keeps the `<sheets>` container out). The counter also counts 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,Line truncated
+
 ### 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.

docs/wiki/Working-With-Files.md

@@ -17,6 +17,7 @@ It renders what it can:
 | Markdown                 | Rendered by default: headings, tables, code blocks with copy buttons, images and links relative to the file (root-relative ones resolve from the workspace root, as on GitHub). Opened from an attachment card, where the file's folder is unknown, relative images show their alt text and relative links show as plain text. The MD pill in the header flips to source. |
 | Images                   | Inline.                                                                    |
 | Audio and video          | Inline with a working scrub bar, because range requests are supported.     |
+| Spreadsheets (`.xlsx`)   | Read-only grid, parsed in your browser (never on the server), up to 10 MB. `.xls` and `.ods` are download only. |
 | PDF and Office documents | Converted for preview when a converter is available.                       |
 | Anything else            | Download.                                                                  |