The existing user_black parameter applies a single black level to all channels, causing color casts when correcting low-value noise. libraw supports per-channel corrections via user_cblack[4] but rawpy didn't expose it.

Changes

• Added user_cblack parameter to Params.__init__() accepting a 4-element list for [R, G, B, G2] channels
• Validation ensures exactly 4 values provided
• Integration with libraw's user_cblack[4] array in apply_params()
• Documentation clarifies that user_cblack values are offsets/corrections applied on top of user_black
• Test coverage for parameter validation, postprocessing, and combined usage with user_black

Usage

import rawpy

with rawpy.imread('image.nef') as raw:
    # Per-channel corrections on top of base black level
    # Effective black levels: [110, 120, 130, 120]
    rgb = raw.postprocess(user_black=100, user_cblack=[10, 20, 30, 20])

Key behavior: user_cblack values are corrections/offsets applied on top of the base user_black value, not absolute black levels. This allows setting a common base black level with per-channel fine-tuning.

Pattern follows existing user_wb parameter. Backward compatible - defaults to [0, 0, 0, 0] when not specified.

• Fixes Support per-channel black level corrections #176

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