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

default.nix

@@ -0,0 +1,39 @@
+# YAPF for Nix / NixOS: the `yapf` tool, libyapf (static + shared) and
+# yapf.h, plus the image/x-yapf MIME type and a freedesktop thumbnailer.
+#
+#   nix-build                 build ./result
+#   nix-env -if .             install for your user
+#   NixOS: environment.systemPackages = [ (pkgs.callPackage /path/to/YAPF { }) ];
+{ pkgs ? import <nixpkgs> { }, lib ? pkgs.lib, stdenv ? pkgs.stdenv }:
+
+stdenv.mkDerivation {
+  pname = "yapf";
+  version = "2.0";
+
+  src = lib.cleanSourceWith {
+    src = ./.;
+    filter = path: type:
+      let base = baseNameOf path; in
+      !(lib.elem base [ "build" "dist" "result" ".git" "node_modules" ]);
+  };
+
+  makeFlags = [ "PREFIX=$(out)" ];
+  doCheck = true;
+  checkTarget = "test";
+
+  postInstall = ''
+    install -Dm644 extensions/linux/yapf-mime.xml $out/share/mime/packages/yapf.xml
+    install -Dm644 extensions/linux/yapf.thumbnailer $out/share/thumbnailers/yapf.thumbnailer
+    substituteInPlace $out/share/thumbnailers/yapf.thumbnailer \
+      --replace-fail "TryExec=yapf" "TryExec=$out/bin/yapf" \
+      --replace-fail "Exec=yapf " "Exec=$out/bin/yapf "
+  '';
+
+  meta = {
+    description = "YAPF: fast lossless images and GPU-ready textures";
+    homepage = "https://github.com/Nexoniarz/YAPF";
+    license = lib.licenses.asl20;
+    platforms = lib.platforms.unix;
+    mainProgram = "yapf";
+  };
+}