[Compat] Fix Windows runtime issues (paths, exec, terminal)

[DecampsRenan]

Why

Internal step — unblocks the rest of the Windows Epic. spm currently makes assumptions that break on Windows: POSIX path separators, POSIX-style command resolution, ANSI-only terminal handling. This sub-issue makes the codebase build and run cleanly on Windows so the audio backend, cross-compile, and installer work has stable ground.

Topics

Path & filesystem handling

• Any hard-coded / separator replaced by filepath.Join / filepath.Separator
• Lock file detection walk-up works from Windows drive roots (C:\)
• Config / cache directories resolved via os.UserConfigDir / os.UserCacheDir

Process execution

• Package manager binaries (npm.cmd, pnpm.cmd, yarn.cmd, bun) resolved and invoked correctly
• Exit codes propagate correctly through cmd.exe / PowerShell
• Ctrl+C cancels child processes cleanly

Terminal & TUI

• TUI renders correctly in Windows Terminal, PowerShell, and cmd.exe
• ANSI colors degrade gracefully on legacy consoles
• Progress spinner clears properly on completion