fix: replace fragile hardcoded stack-frame depth in MBGlobalPackages

[alubbock]

Summary

• capture_functions previously walked exactly 3 frames up the call stack to find the caller's globals. This would silently capture the wrong scope if any internal call chain changed.
• Replaced with a path-based search that walks up until the first frame whose source file is outside the microbench package directory (excluding tests/ so existing tests still work)
• The existing test_capture_global_packages test covers this