Add Pyrefly auto-install and workspace Python interpreter selection

[hendrikmennen]

Summary

• Replace manual pylsp support with pinned native Pyrefly packages for Windows, Linux, and macOS on x64/ARM64, using the existing auto-download preference and package manager.
• Add a workspace interpreter picker with environment detection, browsing, machine-local persistence, and a global default. Report invalid explicit selections instead of silently falling back.
• Apply interpreter changes to workspace-scoped language services, support Python stub files, and fix Python comments and formatter-independent indentation.
• Harden shared LSP activation, restart cancellation, and package-update reactivation; document setup and limitations.

Scope

No pylsp compatibility, Python runtime/environment installation, or Ruff formatting. Pyrefly project configuration can take precedence over the IDE interpreter fallback.

Validation

• 72 Python unit tests passed.
• 14 targeted shared LSP lifecycle regression tests passed.
• Desktop build succeeded.
• Linux x64 Pyrefly smoke coverage exercised completion, hover, navigation, references, rename, and live interpreter switching with spaces/Unicode paths.
• Executable archive paths confirmed for all six package targets; non-Linux-x64 runtime execution was not performed.