Validate zip symlinks and backslash paths before extraction

- treat `\` as a separator in safe_relative so `\abs` and `..\..\x` are refused
- read zip symlink targets with `unzip -p` off the `unzip -Z -l` listing and reject escapes before unpacking

doc/vimpack.txt

@@ -147,14 +147,20 @@ A literal `token` key is rejected at |vimpack-setup| time.
 7. LAYOUT                                                     *vimpack-layout*
 
   stdpath('cache')/vimpack/archives/    downloaded archives
-  stdpath('cache')/vimpack/stage/       extraction staging area
+  stdpath('data')/site/pack/vimpack/.vimpack-stage/   extraction staging area
   stdpath('data')/vimpack/manifest.json installed name/version/sha256
   stdpath('data')/site/pack/vimpack/start/<name>
   stdpath('data')/site/pack/vimpack/opt/<name>
 
 An install extracts to the staging area, hoists a single wrapper directory if
 the archive has one, then renames into place after parking any previous tree
-aside. If the rename fails the previous tree is restored.
+aside. If the rename fails the previous tree is restored. Staging sits under
+the pack root so that rename stays on one filesystem.
+
+Every member is listed and validated before extraction: absolute paths, `..`
+and `..\` escapes, devices, fifos and links pointing out of the destination
+are all refused. Zip symlink targets are read with `unzip -p` (stdout only,
+nothing written) because the zip listing does not carry them.
 
 Lazy loading is out of scope: `opt = true` only chooses the opt/ bucket.