Add pack, cfl and safehtml

pack loads local product packs (format 1): t5de-product.json and the product's files, as product.chkn or
files/, with index.xml's rating, type, gender and parent product. cfl writes and reads IMVU's product
container, uncompressed. safehtml cleans product descriptions by an allowlist (bluemonday).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/pack.md

@@ -0,0 +1,86 @@
+# Product packs (format 1)
+
+A product pack is a folder on the user's machine holding one community product: its manifest,
+`t5de-product.json`, and the product's files. T5DE's proxy serves packs to the client as if they were IMVU
+products. Package `pack` reads and checks them (`pack.Load`).
+
+```
+blue-fur-coat/
+  t5de-product.json
+  product.chkn        the files, zipped (or files/, not both)
+```
+
+## Manifest
+
+`t5de-product.json`, UTF-8 JSON:
+
+```json
+{
+  "format": 1,
+  "id": "blue-fur-coat",
+  "revision": 1,
+  "name": "Blue Fur Coat",
+  "creator": "Some Creator",
+  "description": "<p>HTML description</p>",
+  "rating": "general"
+}
+```
+
+| Field | Rules |
+| --- | --- |
+| `format` | 1 |
+| `id` | 1-64 of `a-z`, `0-9` and `-`, not starting or ending with `-`. Names the product locally; the hub assigns global IDs later. |
+| `revision` | an integer, at least 1. Raise it when the files change, so the client doesn't keep the old ones cached. |
+| `name` | 1-64 characters after trimming spaces, no control characters |
+| `creator` | optional; up to 64 characters, no control characters |
+| `description` | optional; HTML, up to 20000 bytes. Cleaned by an allowlist (package `safehtml`) wherever it's shown. |
+| `rating` | `general` or `adult`; must agree with `index.xml` (below) |
+
+Other fields are an error, and so is anything after the object.
+
+## Files
+
+The product's files are IMVU's product files, as Create Mode makes them: `index.xml` (required), meshes
+(`.xmf`), materials (`.xrf`), textures (`.png`, `.jpg`), and so on. They come one of two ways:
+
+- `product.chkn`: a zip file, Create Mode's project format, with the files at its root. Folder entries are
+  ignored; files inside folders are an error.
+- `files/`: a folder with the files at its top level. Subfolders are an error.
+
+Having both is an error. Other files in the pack's folder are ignored.
+
+Rules for each file:
+
+- Names: not empty, no `/` or `\`, not `.` or `..`, no control characters, Latin-1 characters only (CFL files
+  store names as Latin-1), at most 200 bytes (UTF-8). No two files with the same name.
+- `_contents.json` isn't allowed: it makes the client download a product's files separately, which packs don't
+  support.
+- Limits: 512 files, 64 MiB per file, 256 MiB in all (uncompressed; sizes are counted while reading, not taken
+  from the zip's headers).
+
+`pack.Checksum` hashes the files' names and contents (sha256), so the same files give the same checksum however
+they're stored or compressed. The proxy uses it to notice changes.
+
+## index.xml
+
+IMVU's product template (root element `<Template>`). T5DE reads only these top-level elements and ignores the
+rest:
+
+| Element | Use |
+| --- | --- |
+| `ProductRating` | required: `GA` (general) or `AP` (adult), matching the manifest's `rating` |
+| `ProductName` | the name Create Mode gave it |
+| `ProductGender` | `female`, `male`, ... |
+| `ProductTypeID` | optional integer |
+| `BodyPatternProductId` | optional integer |
+| `__DATAIMPORT` | optional: `product://<id>/index.xml`, the IMVU product it derives from |
+
+A product derives from an IMVU parent product, as every IMVU product does: the client loads the parent's files
+first (from IMVU, as usual) and the pack's files on top. So a pack holds only what it changes, and the parent's
+files never pass through T5DE.
+
+## How the proxy serves a pack
+
+The proxy builds the files into a CFL, IMVU's product container (package `cfl`), uncompressed, files in name
+order, and serves it where the client asks for the product's files. The revision is part of the address the
+client caches the product under, so a new revision makes clients load the new files.