Add optional preprocessing step to marker detection pipeline

[Copilot]

Marker detection preprocessing currently exposes overlapping controls: a preset flag plus per-stage booleans/floats, with preprocessing disabled by default. This change makes "mild" the default preset, renames the stronger presets to descriptive neutral names, and lets None disable preprocessing globally or per stage.

• API Simplification

  • detect_markers(..., preprocess=...) now defaults to "mild".
  • preprocess now accepts str | None; None disables preset preprocessing.
  • preprocess_marker_frame() now uses nullable stage parameters instead of separate boolean toggles:
    • highlight_percentile=None disables highlight clipping
    • clahe_clip_limit=None or clahe_tile_grid_size=None disables CLAHE
    • gaussian_blur_sigma=None disables blur
    • sharpen_amount=None disables sharpening

• Preset Cleanup

  • Kept "mild" as the general default.
  • Renamed "ir" → "strong_highlight_clipping".
  • Renamed "low_light" → "strong_local_contrast".
  • Aligned the "mild" preset with the literal defaults of preprocess_marker_frame().

• Docs and Error Surface

  • Updated marker-detection docstrings to describe the preprocessing parameters directly rather than implying benchmarked lighting scenarios.
  • Clarified the disable semantics for preprocess=None and per-stage None values.
  • Improved invalid-preset messaging to enumerate supported preset names and the disable path.

• Tests

  • Updated preprocessing tests for nullable stage controls.
  • Added coverage for the new default behavior and revised preset/error handling.

video.detect_markers(
    marker_family="36h11",
    preprocess="mild",
    preprocess_params={
        "highlight_percentile": None,
        "sharpen_amount": 0.5,
    },
)

preprocess_marker_frame(
    gray_frame,
    clahe_clip_limit=None,
    gaussian_blur_sigma=None,
    sharpen_amount=None,
)