feat(modules): promote xlsx to builtin module (#86)

* feat(modules): promote xlsx to builtin module

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

* fix(modules): address xlsx review feedback

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

---------

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies

builtin-modules/CLAUDE.md

@@ -24,18 +24,19 @@ Each module should have a standard header comment:
 
 ## Current Modules
 
-| Module | Purpose |
-|--------|---------|
-| `base64` | Base64 encode/decode for Uint8Array |
-| `crc32` | CRC32 checksum calculation |
-| `xml-escape` | XML entity escaping |
-| `zip-format` | ZIP file creation |
-| `str-bytes` | String/bytes conversion |
-| `ooxml-core` | Core OOXML utilities |
-| `pptx` | PowerPoint generation |
-| `pptx-charts` | Chart support for PPTX |
-| `pptx-tables` | Table support for PPTX |
-| `shared-state` | Cross-handler state management |
+| Module         | Purpose                             |
+| -------------- | ----------------------------------- |
+| `base64`       | Base64 encode/decode for Uint8Array |
+| `crc32`        | CRC32 checksum calculation          |
+| `xml-escape`   | XML entity escaping                 |
+| `zip-format`   | ZIP file creation                   |
+| `str-bytes`    | String/bytes conversion             |
+| `ooxml-core`   | Core OOXML utilities                |
+| `pptx`         | PowerPoint generation               |
+| `pptx-charts`  | Chart support for PPTX              |
+| `pptx-tables`  | Table support for PPTX              |
+| `xlsx`         | Excel workbook generation           |
+| `shared-state` | Cross-handler state management      |
 
 ## Workflow