fix TStreamerLoop counter offset in member-wise streaming

[tamasgal]

This Pull request:

Fixes TStreamerLoop counter offset in member-wise streaming and adds a regression test.

Changes or fixes:

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #22895

[dpiparo]

Thanks for this proposal. Just to double-check: have you had a moment to give a look to ROOT's contributing policy? https://github.com/root-project/root/blob/master/CONTRIBUTING.md#ai-assisted-coding-disclosure

[tamasgal]

no LLM involved, only tooling were clang-format and my ROOT binary reverse engineering skills from the past years ;)

[tamasgal]

many thanks for the comments! Meanwhile me and my colleague (with more that two decades of ROOT experience and the original author of our I/O library) are still doing tests and found other issues, so let me put this on hold for now.

I'll implement your suggestions of course but I fear it's not the whole story ;(

[tamasgal]

as mentioned in the upstream issue, we verified the patch also with the extensive high-level test suite, everything looks fine from our side :)