style: Declare data of Stream classes private, instead of protected

[N-Dekker]

Following C++ Core Guidelines, Jul 8, 2025, "Avoid protected data"

[N-Dekker]

@thewtex The CI rejects this PR at PR Checks / Check and lint PR (pull_request), saying:

   type must be lower-case [type-case]
   type must be one of [build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test] [type-enum]

Could this commit check possibly be extended, to allow ITK style commit text?

[thewtex]

👍 @N-Dekker thank you!

type must be lower-case [type-case]
type must be one of [build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test] [type-enum]

These are the conventions of a broadly adopted standard, Conventional Commit:

https://www.conventionalcommits.org/en/v1.0.0/

that is used throughout the web community, and has a bunch of tooling built around it. Since this is a web-oriented project and we use some of that tooling, we use this convention.

[thewtex]

🙏