Implement HALT wakeup interrupt foundation

[Sakery]

Summary

Introduce a minimal interrupt wakeup foundation for the Z88 emulator.

This PR improves Z80 HALT behavior by allowing the CPU to wake from HALT when an interrupt request is received and interrupts are enabled.

The implementation intentionally avoids full Z80 interrupt mode handling for now and focuses on a small, safe, incremental foundation.

Changes

• Implement proper HALT state handling
• Add interrupt request mechanism to Cpu
• Add minimal HALT wakeup path
• Introduce CpuSnapshot-based state inspection
• Improve CPU observability for future testing
• Rewrite README with detailed technical documentation
• Refine trace integration in z88headless

Current behavior

OZ now:

• boots successfully
• initializes BLINK
• enters HALT idle loop
• wakes correctly from simulated interrupt events
• re-enters idle loop coherently

Notes

This is intentionally not a full interrupt implementation yet:

• no IM1/IM2 support
• no interrupt vectors
• no stack push sequence
• no hardware interrupt devices yet

The goal is to establish a stable behavioral foundation for future timer and hardware interrupt work.