[frankschlegel]

This is the first step toward a future Homebrew formula for Frames. The goal is to add official brew install frames-cli support.

Standard Python packaging gives a future formula a way to install Frames
and gives users a pip install option today.

• Add pyproject.toml so pip installs the existing frames executable
  and its Pillow dependency.
• Read the package version from frames at build time so it matches
  frames --version.
• Document installation directly from GitHub. The standalone script,
  symlink installation, and frames setup continue to work as before.

Verification

• Installed from a source archive in a clean virtual environment and
  confirmed that pip installed Pillow and the package version matched
  frames --version.
• Ran frames setup <path> from outside the checkout and checked that it
  saved the expected asset path.
• Ran all 107 tests on macOS with FFmpeg 9.0.2 and oxipng 10.2.1. All passed, including native HEVC alpha and asset-pack trimming.