feat: identify AppImages by reading their squashfs name table

Every AppImage came out as "cannot tell" because the payload is a
compressed filesystem and none of the Chromium markers is on disk to be
found. The names are reachable, though: squashfs keeps the name of
everything it holds in one table of a few kilobytes, and a name is all
this question needs. That table is now read and inflated in place -
nothing is unpacked, and the image is never run, which a scan started by
the path unit has no business doing anyway.

So a Tauri AppImage such as Modrinth is recognised as WebKitGTK and
dropped from the list instead of being offered as something that could be
switched on - there is no autoscroll in WebKit to ask for - and an
Electron one is covered like any other application.

Left as "cannot tell": lzo and lz4, which squashfs stores as bare blocks
no tool will read without their own framing, and the original ISO9660
layout.

Alongside:

- The detect cache carries a format line. Its entries are keyed on size
  and mtime, so an AppImage that has not changed would otherwise keep
  answering the way an older version decided it did, forever.

- A marker list for whole-tree searches, which is what a Flatpak, a snap
  and an AppImage need. icudtl.dat, snapshot_blob.bin and resources.pak
  are out of it: Flutter ships an icudtl.dat in data/, and next to a
  binary those names are evidence while four directories down they are
  not. The Flatpak and snap searches now share that list.

README.md

@@ -117,6 +117,13 @@ New apps are picked up by a systemd path unit that watches the
 relevant directories. If you hate systemd; `middleclick-autoscroll apply` does the
 same thing manually.
 
+AppImages get looked into as well. The payload is a squashfs image glued onto
+the runtime, and squashfs keeps a small table with the name of every file in
+there - so that table gets read and unpacked (a few kb, nothing is extracted
+and the thing is never run) and the Chromium files are looked for in it. An
+Electron AppImage is covered like any other app; a Tauri one is left alone,
+because WebKitGTK simply has no autoscroll to switch on.
+
 ## Commands
 
 | Command | |