fix: enable Butteraugli and SSIMulacra support on Windows

[Adithyakp86]

• Update butter_exe() and ssimul_exe() to check for bundled Windows binaries in butteraugli/windows/ and ssimulacra/windows/
• Add shutil.which() fallback to find tools on system PATH
• Create placeholder windows/ directories with .gitkeep
• Add 15 unit tests for all exe helper functions

#54

Summary

Closes #2 — External Tools (Butteraugli, SSIMulacra) Not Supported on Windows.

butter_exe() and ssimul_exe() in gui/utility.py previously returned None on Windows, completely disabling these image quality metrics. This PR adds full Windows support.

Changes

gui/utility.py

• Updated butter_exe() to check for a bundled binary at butteraugli/windows/butteraugli.exe
• Updated ssimul_exe() to check for a bundled binary at ssimulacra/windows/ssimulacra.exe
• Added shutil.which() fallback for both functions — if no bundled binary is found, the system PATH is searched
• Added import shutil at the top of the file

New Directories

• gui/butteraugli/windows/.gitkeep — placeholder for bundled butteraugli.exe
• gui/ssimulacra/windows/.gitkeep — placeholder for bundled ssimulacra.exe

tests/unit/test_utility.py

• Added 15 new unit tests for butter_exe(), ssimul_exe(), and exiftool_exe() covering:
  • Bundled binary detection (Linux & Windows)
  • System PATH fallback (Linux & Windows)
  • Not-found returns None
  • Unsupported platform returns None

How It Works

Resolution follows a two-step process (matching the existing exiftool_exe() pattern):

1. Bundled binary — check for .exe in the windows/ subdirectory
2. System PATH — fall back to shutil.which() to locate user-installed tools

Testing

All 33 tests passed (18 existing + 15 new):

[Adithyakp86]

once check this PR and merge