YAPF v1: fast byte-aligned format, SIMD, tools and plugins

Format (version 1, replaces the earlier experimental versions):
- per-tile YCoCg-R or subtract-green colour transform, 8-bit modular
- 12 row filters per 8-row band (Left, Gradient, Up-right, Up-average,
  Up 1-8 rows), none with more than a one-add serial dependency
- residuals in groups of 8 at the group's width (w bits x 8 = w bytes),
  4-bit control codes with repeat-previous-width runs chosen by DP
- constant and stored tiles, independent 64x64 tiles, mips, GPU hints
- all sizes validated before decoding; corrupt input is rejected

Library:
- SIMD on by default: SSSE3 (runtime CPUID check; GCC, Clang, MSVC,
  MinGW) and NEON on ARM64; -DYAPF_NO_SIMD for plain C
- yapf_load_memory, yapf_load_mt, yapf_load_memory_mt
- 6-10x faster decoding than PNG (stb_image) on one core; -O2 and -O3
  builds perform the same
- fixes for macOS (sysconf) and the Windows import library exports

Tools and integrations:
- `yapf` CLI: convert PNG/JPG/TGA/BMP/PSD/GIF <-> YAPF, info, bench,
  thumbnail, mips
- JavaScript and Python bindings (byte-identical to the C encoder)
- GIMP 3 plugin: exports the flattened image, optional mipmaps
- Blender extension: import (also as plane), export, drag and drop
- VS Code preview, Figma plugin, Photoshop scripts, Linux MIME type and
  thumbnailer
- Makefile, updated build.sh / build.bat, tools/build_release.sh
- tests/test_yapf.c (784 checks) and bindings/js/test.js

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

bindings/python/README.md

@@ -0,0 +1,23 @@
+# yapf.py — YAPF in Python
+
+```python
+import yapf
+
+img = yapf.load("texture.yapf")        # or yapf.decode(data)
+print(img.width, img.height, img.channels, img.mip_levels)
+img.pixels                             # bytes, rows top to bottom
+
+yapf.save("copy.yapf", img)            # or yapf.encode(img) -> bytes
+yapf.save("new.yapf", yapf.Image(w, h, 4, rgba_bytes))
+```
+
+It uses the C library through `ctypes` when it finds one:
+
+1. the file named by the `YAPF_LIBRARY` environment variable,
+2. `libyapf.so` / `libyapf.dylib` / `yapf.dll` next to `yapf.py`, or in
+   `lib/<platform>/` beside it (e.g. `lib/windows-x64/yapf.dll`),
+3. the system library path (`sudo make install` installs one).
+
+Without the library it falls back to a pure-Python decoder that needs
+`numpy` (decoding only; about 1.4 s for a 4K image).  `yapf.backend()`
+tells you which one is in use.