Optimize PowerShell test runners

[gloveboxes]

Summary

Optimizes the PowerShell runner and its child test suites while retaining test coverage, per-run build isolation, and safe timeout/process-tree cleanup.

• Reuse helper-function setup in parallel runspaces instead of rebuilding it for every app/mode dispatch.
• Cache parsed performance baselines for fail-fast checks instead of rereading the CSV for every result.
• Suppress passing per-test output inside the extended, diagnostics, and dccpeep child runners when the parent is running in failure-only mode.
• Run the independent dccpeep fixture cases in parallel at the configured throttle limit, retaining both the baseline comparison and idempotence pass.
• Preserve the extended suite’s existing overlap with diagnostics and dccpeep checks.

Validation

• PowerShell syntax parsing passed for every changed script.
• 54 dccpeep fixtures passed at throttle limits 1 and 16.
• 116 diagnostics passed with failure-only output.
• Full -Mode full -FailFast -FailuresOnly main suite passed with no performance regressions.
• Full -Mode full -Extended -FailuresOnly run passed: 483 main apps and extended tests, no performance regressions.

The local full-run measurement was 14.86s after these changes; use repeated alternating benchmarks for performance conclusions because normal host variability is material.