fix: preserve selected subtitle colors

[LauraGPT]

Summary

• render subtitle text with Pillow RGBA instead of MoviePy 1.0.3's external ImageMagick TextClip path
• preserve the existing SubtitlesClip timing, bottom-center placement, clipping, and FFmpeg video encoding
• resolve the bundled subtitle font relative to the package and declare Pillow as a direct runtime dependency
• add pixel-level coverage for black, white, green, and red, plus a composed-video-frame regression

Context

Related to #135. Two users reported that every subtitle color choice exported as white. The UI value already reached TextClip(color=font_color), so this change removes the external rasterizer from that contract and produces a transparent RGBA clip with the selected color directly.

This PR deliberately does not close #135. The issue should remain open until a reporter verifies an exported video in their original environment.

Verification

• red test: the new renderer test failed to collect on exact main because no deterministic subtitle renderer existed
• 24 passed across subtitle rendering, release contract, recognition compatibility, and duplicate-text matching
• py_compile passed for the renderer and video clipper
• pip check reported no broken requirements on MoviePy 1.0.3 / NumPy 1.26.4 / Pillow 12.3.0
• git diff --check passed
• one-second MP4 export and decode contained 1,297 red subtitle pixels and 0 white pixels

Exact signed+DCO head: c0d390d238b539030719533e9b702872284be3d6.