[Action] fix report-only variable actions

[Harmenszoon]

Summary

• Treat variable,op=report as a report configuration action instead of an executable APL action.
• Handle OPERATION_REPORT as a no-op if it ever reaches variable_t::execute().

Details

op=report only enables sample-sequence reporting for the named variable. Before this change it was still added to the foreground action list, so a standalone report action could execute and hit the debug assert(0) path in variable_t::execute().

Marking the action as background matches the existing action contract for actions that should not be directly executed from an action list, and keeps the report-only action out of variable mutation/constant tracking.

Testing

• Built with local CMake/Ninja Visual Studio configuration.
• simc.exe build-codex\variable_report_repro.simc iterations=1 max_time=1 output=NUL cleanup_threads=1
• simc.exe build-codex\variable_report_sequence.simc iterations=1 max_time=2 report_details=1 output=build-codex\variable_report_sequence.txt html=build-codex\variable_report_sequence.html cleanup_threads=1
• Verified the generated HTML sample sequence contains VAR [set] for the reported variable.
• simc.exe profiles\CI.simc iterations=10 output=build-codex\ci_variable_report_validation.txt cleanup_threads=1
• git diff --check