Harden self-hosted Runtime installation and interrupted recovery

[SaladDay]

Self-hosted Runtime installation currently fails immediately on transient downloads and leaves interrupted staging behind. Bound and retry downloads, check available space, report useful filesystem errors, and recover unfinished staging without replacing completed components, credentials or workspaces. Add terminal progress and preserve plain redirected output.

Validation:

• Focused CLI, native bootstrap and runtime filesystem suites pass on macOS arm64 and Linux amd64 (ordinary user and root).
• Regression coverage exercises transient HTTP failures, truncated downloads, corrupt archives, low space, stale staging, concurrent recovery, parent-process death, and paths containing spaces.
• Native platform CI passes on Linux amd64, macOS arm64 and Windows amd64, including download recovery, packaged installation, authentication and connection.
• Distribution documentation checks and repository name checks pass.

Independent blind review of the full diff passed after the identified issues were fixed and covered by regression tests. Native CI passed on the final commit (be18a6deaf70d3a55a7f359474c330914afbc95a). Managed sandbox startup, node installation and Core/Web installation are outside this change.