Consider a -WhatIf/dry-run mode for Invoke-OverlappingPoolTests.ps1 so developers can see the schedule without spawning six suites. #140

What

Add a -WhatIf (or equivalent dry-run) switch to .github/scripts/Invoke-OverlappingPoolTests.ps1 that prints the planned run schedule — the sequence of overlapping runs, their start offsets, and the suites each run would spawn — without actually launching any pwsh/powershell child processes.

Why

The script currently spawns six overlapping test suites whenever it is invoked, which makes it expensive and disruptive to run locally just to inspect or sanity-check the schedule. A dry-run mode lets developers (and reviewers) verify the timing/overlap logic and the run plan cheaply, without the side effects of spawning real processes. This was raised as a non-blocking follow-up during the AI review of PR #124.

How

• Add a [switch]$WhatIf parameter to Invoke-OverlappingPoolTests.ps1.
• When $WhatIf is set, compute the same schedule the script would use (run order, start offsets, deadline) but skip the process-spawning calls (the Start-Process/Register-Spawned path and the per-run suite invocation).
• Emit the planned schedule to the host (e.g. one line per run: index, suite, planned start offset, planned end/deadline) so it is readable in CI logs and locally.
• Ensure the script exits with a success code in -WhatIf mode and does not attempt overlap detection, timeout handling, or report generation that depends on real process results.
• Keep the default (no switch) behavior byte-for-byte identical to today.

Files Affected

• .github/scripts/Invoke-OverlappingPoolTests.ps1
• Unknown (any test that asserts on Invoke-OverlappingPoolTests.ps1 behavior, if one exists)

Constraints

• Must not change default behavior when -WhatIf is not passed.
• Must not spawn any child processes, write .pid files, or register tracked processes in dry-run mode.
• Must not alter the existing overlap-detection, timeout, or reporting logic for real runs.
• Out of scope: changing the schedule itself, adding new suites, or modifying Stop-TrackedProcesses / Register-SlotPidFile / Get-StandInCommand.

LLM tier

Sonnet — moderate design judgment: adding a mode flag to an existing script while preserving current behavior and deciding what the dry-run output should contain.

Success looks like

• ./.github/scripts/Invoke-OverlappingPoolTests.ps1 -WhatIf prints the planned schedule and exits 0.
• No child pwsh/powershell processes are spawned and no .pid files are written during -WhatIf.
• Running the script without -WhatIf produces the same behavior as before the change.
• The printed schedule matches the runs the script would actually perform.

Failure looks like

• -WhatIf still spawns processes or writes .pid files.
• Default (no-switch) behavior changes.
• The dry-run output diverges from the real schedule (e.g. wrong run count or offsets).
• The script errors or exits non-zero in -WhatIf mode.

Follow-up from AI review of PR #124.