Update to OTIO 0.18.1

[jminor]

This PR updates OpenTimelineIO-Swift-Bindings to use the latest released OpenTimelineIO 0.18.1 (was 0.17.0). This includes:

• Updating to OTIO v0.18.1
• Marking RationalTime.isValidTimecodeRate as deprecated in favor of RationalTime.isSmpteTimecodeRate
• Fixing an integer overflow typo.
• Switching CI to use macos-26 instead of macos-latest.

[jminor]

The build is failing because it needs the version.h files which are normally generated by CMake, but that step doesn't happen via Swift build... It was working on my machine because it found the installed /usr/local/include/opentime/version.h files. I'd rather not force folks using this Swift package to have CMake installed if we can avoid it.

Question for the OTIO community: Could we get by with a stub version.h file in the bindings repo to get around this? Do we need the correctly versioned namespace when using OTIO via Swift bindings?

[camkerr]

Thanks for this update @jminor

I tested this locally with my "Cutlist" macOS app's test suite and had Claude make a quick little app to test RationalTime.isSmpteTimecodeRate using some sample .otio files of mine from Media Composer.

Looks good, everything passes on my end.

[camkerr]

I also brought it into the OpenTimelineIO-AVFoundation project and the included OpenTimelineIO-Reader sample macOS app and everything works as expected.

[camkerr]

Double checked this in Xcode. It shows deprecated as expected.