Improve viewer core stability and search performance

[gernotmuc]

Summary

• combine the public core fixes into one additive base PR
• stabilize search result handling and plugin configuration reloads
• preserve ECU tree contexts across repeated GetLogInfo merges and add regression coverage
• speed up the Qt6 search hot paths in qdlt and the search dialog

Why

• each PR state should build on its own
• the later filter work can stack on this branch instead of competing with it

Notes

• this is intended as the standalone core/base layer for the follow-up filter PR
• validation for this stack was done as a standalone build and as the base for the next layer

[gernotmuc]

Merge guidance for maintainers:

1. This is the intended first merge in the public restack.
2. The old parallel public PRs (Guard plugin selection refresh during config reload #808, Fix/config ctid merge saveidscsv clean #811, Speed up Find All search in viewer #813, Fail safely on malformed DLT filter files #815, Short-circuit filter rejects before decode #817) were closed as superseded so this PR is the only upstream base layer.
3. The filter follow-up is intentionally kept separate in Improve filter loading and predecode handling gernotmuc/dlt-viewer#9 so the second layer stays reviewable on top of this one instead of competing with it.
4. After this PR is merged into COVESA/master, the remaining action is to take the delta from Improve filter loading and predecode handling gernotmuc/dlt-viewer#9 and publish it as the second upstream PR against the updated master.

So the intended sequence is: merge #818 first, then land the filter layer as the follow-up step.