make _test-vsix cannot go green on Windows — Linux-shaped staging assertions + temp-dir EPERM cleanup flakes

Running the full make _test-vsix gate on Windows 11 (the primary .NET dev OS) has pre-existing platform failures unrelated to feature work:

1. Extensionless sidecar filename assertions (3 tests)

• bundled-sidecars.test.ts 'sidecars are present in bin/all/' asserts bin/all/sharplsp-sidecar-csharp — on Windows staging produces sharplsp-sidecar-csharp.exe (Makefile EXE_EXT), so existsSync fails.
• 00-vsix-dev-binary-staging.test.ts 'keeps both required sidecars bundled...' — same class.
• bundled-binary.test.ts 'sharplsp resolves from bundled source' fails downstream of the same staging-shape mismatch.

These pass in CI because ci.yml runs the gate on ubuntu only. Fix: assert platform-correct names (process.platform === 'win32' ? name + '.exe' : name), as bundled-binary.test.ts already does for the LSP binary itself.

2. after-hook EPERM temp cleanup flakes (2+ tests)

Error: EPERM, Permission denied: \?\C:\Users\...\Temp\sharplsp-debug-cmd-e2e-qGlWOA
Error: EPERM, Permission denied: \?\C:\Users\...\Temp\sharplsp-workflow-PS5Gjv

Windows holds file locks (watchers/processes) while suite teardown rmSyncs the fixture tree — same failure class as the project-deps watcher crash fixed earlier (see VSCODE-REACTIVITY-SPEC §4 fix). Teardowns need retry-on-EPERM (rmSync with maxRetries/retryDelay) and/or disposing watchers before deletion.

Evidence

Full gate on Windows: 539 passing, 16 failing — of which only 1 is an intentional tracked repro (#160). The rest split across the two classes above plus in-flight local edits. Log excerpt available on request.

Windows is where .NET developers live — the gate should be green here, not just on ubuntu CI.

Start by running the Windows make _test-vsix gate and inspect bundled-sidecars.test.ts, 00-vsix-dev-binary-staging.test.ts, and bundled-binary.test.ts, using the existing platform-aware LSP assertion as a reference. Then trace the after-hook cleanup for the debug-command and workflow fixtures, and review VSCODE-REACTIVITY-SPEC §4. Done means the gate passes on Windows without breaking the Ubuntu assertions, including reliable temporary-directory cleanup.