Carry RBX through validated PVH loading

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A lightweight x86 guest should be able to enter through the PVH boot protocol
without a general-purpose firmware boot. That requires both validated image loading
and complete initial-register plumbing: the boot-information address carried in
RBX must reach the virtual processor rather than being lost between abstractions.

Proposed Scope

• Carry the required RBX value through the image-loader, initial-state, and
  supported hypervisor interfaces.
• Validate PVH image metadata, segment ranges, entry state, and boot-information
  placement before writing guest memory or starting a processor.
• Reject malformed images, unsupported layouts, and address arithmetic overflow
  with contextual errors, without panicking on input.
• Keep register plumbing and the PVH loader in one working change so the entry
  contract is testable end to end.
• Preserve other boot paths and use guest-architecture gating for guest-specific
  behavior.

Acceptance Criteria

• A valid PVH image produces the expected entrypoint, boot-information address,
  and register state, including RBX.
• Loader fixtures verify segment contents and address bounds.
• Truncated metadata, invalid ranges, and overflowing addresses fail before
  guest execution without out-of-bounds writes or panics.
• Initial-state tests verify RBX reaches each supported backend and that other
  boot modes retain their register behavior.
• The accepted PVH contract and unsupported cases are documented.

Dependencies

No feature prerequisites. The cold-boot microVM profile will consume the loader
and register plumbing together.

Non-Goals

This issue does not add the full machine profile, device model, CLI mode, or snapshot
support.