Fix Windows installer entrypoint execution

[austin-smith]

What Changed

• replace invocation-context detection with an explicit -LoadOnly switch for installer unit tests
• run the public Windows installer smoke test in a clean Windows PowerShell process
• add regression coverage for evaluation from a dot-sourced scope and for the release workflow command

Why

The v0.2.0 release smoke test evaluated the downloaded installer inside GitHub Actions' dot-sourced PowerShell scope. $MyInvocation.InvocationName was therefore ., so the installer silently skipped Install-Spotuify and the workflow failed when it looked for the missing executable.

Failed run: https://github.com/austin-smith/spotuify/actions/runs/30894354563/job/91947662940

Validation

• bun test — 1,041 passed
• bun run typecheck
• bun run installer:test:posix — 19 passed
• focused release workflow tests — 3 passed
• structured autoreview — clean
• git diff --check