fix(oiiotool): better type understanding with -i:ch= and other cleanup

[lgritz]

Background: oiiotool is pretty good at remembering the data type of the first file it reads in a command, and ensuring that the output preserves the same types, despite the fact that internally, it tends to keep everything in float. (As a further reminder: -d can override the heuristics that make this work.)

But there was an exception: when -i was used with the optinal modifier -i:ch=... to select a subset of channels or reorder channels upon read, the underlying call to IBA::channels would cause it to not have that information in the "nativespec" at the right time that it was collected.

This PR primarily ensures that the -i:ch=... preserves the information about which channels had which data types, despite the channel shuffle.

Some other bits and pieces along the way:

• A new TypeDesc::basetype_merge() variety that take a cspan<TypeDesc>.
• A new utility TypeDesc::all_types_equal(cspan<TypeDesc>)
• Cleanup in IBA::channels() to more correctly compute the common type of the channel subset that you actually specify.

[Copilot]

Pull request overview

This PR fixes oiiotool’s native type propagation when -i is combined with channel selection/reordering (-i:ch=...), ensuring the “first input’s” per-channel native types are preserved even after a channel shuffle.

Changes:

• Preserve/restore nativespec channel format information after applying ImageBufAlgo::channels() during -i:ch=....
• Add new TypeDesc helpers: basetype_merge(cspan<TypeDesc>) and all_types_equal(cspan<TypeDesc>).
• Update ImageBufAlgo::channels() to compute the merged output format from the actually-selected channels and clear channelformats when uniform; extend tests to cover the regression case.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
testsuite/oiiotool-copy/run.py	Adds a regression test exercising channel reordering/renaming without -d to validate correct type propagation.
testsuite/oiiotool-copy/ref/out.txt	Updates expected output to include info for the new regression output file.
src/python/py_typedesc.cpp	Exposes TypeDesc::all_types_equal to Python.
src/oiiotool/imagerec.cpp	After IBA::channels() channel shuffles, restores per-channel native types into nativespec and recomputes merged native format.
src/libOpenImageIO/imagebufalgo_channels.cpp	Computes newspec.format from selected channel formats and clears per-chan formats when all equal.
src/include/OpenImageIO/typedesc.h	Adds span-based basetype merging and uniform-type checking helpers.

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[lgritz]

2 weeks, no objections. Will merge by the end of the week.