refactor(cpu): add unknown opcode tracking infrastructure

[Sakery]

This is a minimal, observability-only change that enables z88headless to collect and report unknown opcodes without modifying core emulation semantics.

Changes:

• Add UnknownOpcode struct to track PC and opcode byte
• Add m_unknownOpcodes vector to accumulate all unknown opcodes
• Add m_traceUnknown flag for runtime control of output
• Add setTraceUnknown() public method for controlling trace
• Add unknownOpcodes() getter for accessing collected data
• Add recordUnknown() private method to handle both tracking and optional printing
• Refactor CB prefix unknown handling to use recordUnknown()
• Refactor ED prefix unknown handling to use recordUnknown()

Rationale:

• No opcode behavior changes: all dispatch logic unchanged, only tracking added
• No emulation semantics changes: instruction execution identical to before
• Unknown opcodes are passively collected; printing is opt-in via setTraceUnknown()
• This enables z88headless to provide detailed diagnostics for regression testing
• Memory overhead is minimal (two bytes per unknown opcode encountered)

Summary

Describe clearly what this PR changes.

Checklist

• Project builds successfully
• Tests pass locally
• CI passes
• New logic includes tests where possible
• No ROM binaries committed
• No warnings introduced

Technical notes

Explain implementation details, tradeoffs or TODOs.