[Installer] PowerShell install script

[DecampsRenan]

Why

Windows users need a one-liner to install spm without having Go on their machine, mirroring the existing curl | bash flow on macOS/Linux. Blocked by [Build] (the script downloads Windows binaries from GitHub releases).

Topics

Script behavior

• install.ps1 downloads the latest release archive matching the user's architecture
• Extracts the binary to a user-writable directory (e.g. %LOCALAPPDATA%\spm)
• Adds that directory to the user's PATH
• Env var or parameter lets the user pin a specific version

Robustness

• Downloaded archive verified against checksums.txt
• Clean, actionable error message on download / extraction failure
• Re-running the script upgrades in place

Hosting & invocation

• Script lives at scripts/install.ps1 and is fetchable via raw.githubusercontent.com
• One-liner iwr <url> -useb | iex works in PowerShell 5.1 and PowerShell 7