Cleanup: Use enum class

[hnakamur]

This pull request converts non-class enums to enum classes.
There are still many non-class enums not addressed in this pull request.
However, since the changes have grown quite large, I'm submitting it at this point.

[Copilot]

Pull Request Overview

Converts traditional enums to strongly-typed enum class across headers and documentation, updating member types, function signatures, and references to use scoped enum values.

• Replaces nearly all non-class enums with enum class variants (some with explicit underlying types).
• Updates fields, parameters, function return types, and bitmask flag operations to use the new enum classes.
• Refreshes documentation and API examples to reference the scoped enum names.

[bneradt]

lgtm. Thank you for the improvement.