Add slit-image videogram mode with selectable sampling lines

[Copilot]

Videograms previously only supported axis-averaged rendering. This update adds a photo-finish/slit mode that samples one row or one column per frame over time, with explicit line selection.

• API: videograms slit mode

  • Extended MgVideo.videograms(...) with:
    • mode: "average" (existing behavior) or "slit"
    • line_x: sampled column index for vertical slit videogram
    • line_y: sampled row index for horizontal slit videogram
  • average mode behavior remains unchanged.
  • In slit mode, output filenames default to _vgv_slit.png / _vgh_slit.png.
  • Added slit-mode validation for mode, line_x, and line_y bounds.

• FFmpeg pipeline updates

  • Implemented slit extraction using per-frame crop + tile (time stacking).
  • Added RGB conversion before 1-pixel crop to avoid YUV subsampling crop constraints.

• CLI support

  • Extended musicalgestures videograms with:
    • --mode [average|slit]
    • --line-x
    • --line-y

• Targeted test coverage

  • Added tests for slit output creation, expected dimensions, input validation, and average-mode compatibility when line args are provided.

import musicalgestures as mg

v = mg.MgVideo("input.mp4")
# photo-finish style videograms from chosen column/row
out = v.videograms(mode="slit", line_x=120, line_y=240)
print([img.filename for img in out])  # *_vgv_slit.png, *_vgh_slit.png

• Fixes Add photo finish image option to videogram #365