Extensions read YAPF 2; Nix package

- KDE thumbnailer: texture loader, decodes only the mip it needs; GPU
  blocks, 16-bit and float via the new yapf_tex_to_rgba8. Tested through
  KDE's plugin loader with KF6 from nixpkgs.
- Photoshop: shows the tool's error message; HDR textures open as 32-bit
  (.hdr), 16-bit stays 16-bit; 16-bit documents save as 16-bit.
- CLI: 16-bit PNG export, readable conversion errors.
- default.nix: yapf tool, library, header, MIME type and thumbnailer
  (absolute store path); runs the test suite.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DVrVthWN8GsmgHGUhhsCYm

extensions/linux/README.md

@@ -13,3 +13,22 @@ on your `PATH` — `sudo make install` puts it in `/usr/local/bin`).  They work
 in file managers that use freedesktop thumbnailers: GNOME Files (Nautilus),
 Nemo, Caja, and Thunar (with tumbler).  KDE Dolphin uses its own plugin
 system and is not covered yet.
+
+Thumbnails cover every YAPF file the tool reads: version 1 and 2, GPU
+blocks (BC1–BC7, BC6H), 16-bit and float textures, and the first image of
+cubes, arrays and animations.  KDE Dolphin: see [../kde](../kde).
+
+### NixOS
+
+`/usr/share` is not used on NixOS.  Install the package from the
+repository's `default.nix` instead — it carries the MIME type and a
+thumbnailer that calls `yapf` by its full store path:
+
+```nix
+# configuration.nix
+environment.systemPackages = [ (pkgs.callPackage /path/to/YAPF { }) ];
+environment.pathsToLink = [ "/share/thumbnailers" ];   # if your desktop does not link it already
+```
+
+For your user only: `nix-env -if /path/to/YAPF`, then
+`sh extensions/linux/install.sh` (the per-user install).