install: Add explicit offline deployment mount API

[cgwalters-bot]

Post-install tooling needs a backend-neutral way to inspect and modify an offline deployment without exposing backend-specific paths through bootc status. This extracts the mount API work from the composefs stabilization series and makes it standalone on current main.

The reduced API adds bootc install mount for OSTree and native composefs targets. The deployment root remains read-only; --writable makes only /etc and /var writable. Mounts remain in the caller's namespace, and callers clean them up with umount -R or by ending that namespace. Partial assembly failures reuse the existing recursive-unmount helper.

Composefs mounting opens the existing repository directly, follows its durable fs-verity policy, and does not implicitly initialize or upgrade repository metadata.

Validation completed locally:

• affected locked Rust tests and checks;
• no-default-features and all-features checks;
• Nushell static validation;
• CLI and generated manpage consistency;
• formatting and diff/log checks; and
• independent code and architecture review.

This remains a draft while the revised privileged OSTree and composefs TMT coverage runs in CI. The change is larger than 500 lines and should receive explicit maintainer design review before being marked ready.

Assisted-by: AI
Unit and integration tests are LLM generated.