Add shot_select support with set_unpack_params() method

[Copilot]

shot_select was incorrectly implemented as a postprocessing parameter. It must be set before unpacking since it controls which RAW image is decoded from multi-image files (e.g., Canon Dual Pixel RAW).

Changes

• Added shot_select to imread(): High-level API takes shot_select parameter
• Created set_unpack_params() method: New method in RawPy class for configuring unpack parameters
• Set at correct time: imgdata.rawparams.shot_select is set after opening, before unpack() is called
• Removed from postprocessing: Removed from Params class and apply_params()

API Change

Before:

with rawpy.imread('file.cr2') as raw:
    rgb = raw.postprocess(shot_select=1)  # Wrong: too late, after unpacking

After (high-level API):

with rawpy.imread('file.cr2', shot_select=1) as raw:
    rgb = raw.postprocess()  # Correct: set before unpacking

After (low-level API):

raw = rawpy.RawPy()
raw.open_file('file.cr2')
raw.set_unpack_params(shot_select=0)
raw.unpack()
rgb = raw.postprocess()
raw.close()

Technical Details

shot_select lives in imgdata.rawparams (unpacking stage), not imgdata.params (postprocessing stage). The new set_unpack_params() method provides a clean separation between file opening and unpack configuration, making the API more extensible for future unpack parameters. Setting it before unpacking allows LibRaw to properly select the image during decode and correctly error on nonexistent images.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Can we support shot_select?</issue_title>
<issue_description>For instance a Canon RAW with Dual Pixel Raw enabled would have 2 RAW images in the file chosen by shot_select=0 or shot_select=1. Cameras like the 5D Mark IV or EOS R have that feature.</issue_description>

<agent_instructions>See https://www.libraw.org/docs/API-datastruct-eng.html for shot_select. Search for an example RAW image online that has two or more shots - we need to test it. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Can we support shot_select? #175

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