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/photoshop/README.md

@@ -25,6 +25,16 @@ native plugin.
 
 Instead of copying `yapf` next to the scripts you can put it on your `PATH`.
 
+## What opens how
+
+Every YAPF file the `yapf` tool reads (version 1 and 2): 8-bit images open
+as 8-bit documents, 16-bit textures as 16-bit documents, half / float and
+BC6H HDR textures as 32-bit documents (through Radiance `.hdr`), other GPU
+blocks (BC1–BC7) decoded to 8-bit.  Cubes, arrays and animations open their
+first image.  When a file cannot be opened, the alert shows the tool's
+reason (for example a damaged file).  Saving keeps 8 or 16 bits per
+channel.
+
 ## Status
 
 The scripts are written for Photoshop CC's ExtendScript and are not yet