Add x2APIC detection and initialization support #183

Summary

Detect whether the processor and platform support x2APIC, then provide a controlled initialization path alongside the existing interrupt-controller abstraction.

Scope

• Check CPU feature availability before accessing x2APIC-specific MSRs.
• Define the transition and fallback policy between xAPIC and x2APIC modes.
• Validate APIC identifiers and preserve a consistent interface for interrupt routing.
• Report unsupported or failed initialization without leaving the controller partially configured.

Acceptance criteria

• Feature-detection logic is independently testable using synthetic CPU capability inputs.
• Initialization refuses unsupported CPUs and documents required platform prerequisites.
• Existing xAPIC/PIC behavior remains available as the configured fallback.
• Document register/MSR access invariants and mode-selection policy.

Engineering constraints

• Keep the kernel Rust-first, no_std, and x86_64-compatible.
• Integrate with existing Rustalis subsystem interfaces; do not create a parallel implementation.
• Document ownership, initialization order, failure behavior, and any architecture-specific unsafe invariants.
• Add host-side tests for hardware-independent logic and QEMU coverage for hardware paths where practical.
• Do not claim verification that has not actually been run.