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

yapf.h

@@ -601,6 +601,16 @@ int yapf_tex_compress(const yapf_texture_t *src, uint8_t format,
 int yapf_blocks_decode(uint8_t format, const void *blocks, uint32_t w, uint32_t h,
                        void *out);
 
+/*
+ * yapf_tex_to_rgba8 — one slice of a decoded surface as 8-bit RGBA for
+ * display (thumbnails, previews): gray is expanded, 16-bit is rounded to
+ * 8 bits, float and half are clamped to 0..1 and sRGB encoded (they are
+ * linear), GPU blocks are decoded.  out holds w × h × 4 bytes of that
+ * mip.  Returns YAPF_ERR_UNSUPPORTED for ETC2 / EAC / ASTC blocks.
+ */
+int yapf_tex_to_rgba8(const yapf_texture_t *tex, uint32_t surface, uint32_t slice,
+                      uint8_t *out);
+
 /* yapf-sum, the checksum used by version 2 files. */
 uint32_t yapf_checksum(const void *data, size_t size);