fix installer curl fallback

[shayne-snap]

Summary

#137

• fall back to Invoke-WebRequest when Windows curl.exe exists but fails during installer downloads
• remove any partial curl output before retrying with Invoke-WebRequest

Root cause

The PowerShell installer only used Invoke-WebRequest when curl.exe was missing. On Windows environments where curl.exe uses Schannel and fails certificate revocation checks with CRYPT_E_REVOCATION_OFFLINE, the installer stopped even though Invoke-WebRequest could download the same release asset successfully.

Validation

• git diff --check
• make test
• Parallels Windows 11: loaded the modified installer, injected a fake curl.exe that exits 35 and writes a partial destination file, then verified Download-File warned, retried with Invoke-WebRequest, and downloaded the real v0.1.18 checksums.txt (1292 bytes).