Fix Windows Runner exactly-one primary recheck

[yyjeqhc]

Summary

• force array semantics for the final Windows Runner primary-count recheck so Windows PowerShell 5.1 does not scalar-unroll the healthy exactly-one result
• add focused regression coverage in the existing Windows Runner readiness test

Validation

• powershell.exe -NoProfile -NonInteractive -ExecutionPolicy Bypass -File scripts\test_windows_runner_readiness.ps1
• git diff --check
• cargo build --profile dogfood --bin webcodex-runner -p webcodex-runner
• candidate identity: c40aa52734f2 / dirty=false

MSI dogfood

Real replacement completed successfully through the existing deployment helper:

• old instance: 538bd437-ba03-4512-bf17-d8db14d33e62, build b46ece5ef747 / dirty=false
• new instance: 7cdec7c3-687d-4826-9c70-abb291c6608c, build c40aa52734f2 / dirty=false
• readiness: exact_fresh_build_ready
• expected and observed build identities matched exactly
• post-deployment lifecycle: Scheduled Task Running, RunLevel=Limited, exactly one primary Runner, zero unexpected primaries
• temporary dogfood controller/task files were removed; the standard rollback binary was retained by the successful deployment helper

The bounded failure/rollback path was also exercised during the preceding #97 dogfood and proved a fresh rollback Runner before this fix was developed.

origin/main advanced during dogfood only by #161, whose changed files do not overlap these Windows scripts. This branch intentionally preserves the exact dogfooded commit rather than rewriting its history after validation.

Fixes #97