Reset experimental mode and debug info selection in CompilerStack::reset()

[zexoverz]

Fixes #17004. reset(false) reassigned every other setting in the !_keepSettings block but skipped m_experimental and m_debugInfoSelection, so a stack reused after setExperimental(true) or selectDebugInfo(DebugInfoSelection::None()) kept those into the next compilation. Both now go back to the header defaults, false and DebugInfoSelection::Default(). I went through the remaining public setters and did not find another member the block misses.

There is no getter for either, so the two new tests in SolidityCompiler.cpp observe them the way a caller would: the settings.experimental key in the metadata, and the source location comments in assemblyString. Each checks reset(true) keeps the value and reset(false) drops it, and each fails on develop without the change.

Checklist

• I have read the contributing guidelines and the review checklist
• I have personally reviewed, understood, and tested every change in this PR

AI Disclosure

• No AI tools were used

Claude Code was used to draft the fix, the two unit tests and the Changelog line. I reviewed the diff, built soltest locally and ran the SolidityCompiler suite against it.