feat(cpu): memory-mapped I/O hooks

[relmer]

Add a callback/hook mechanism for memory-mapped I/O address ranges.

Motivation: Prerequisite for any platform emulation (NES PPU, C64 VIC-II/SID, Apple II). Also enables simple virtual peripherals for standalone programs (e.g., a character output port).

Proposed API:

• \Cpu::RegisterReadHook(Word start, Word end, ReadCallback)\
• \Cpu::RegisterWriteHook(Word start, Word end, WriteCallback)\
• Hooks intercept reads/writes to the specified address range
• A simple built-in hook: character output device at a configurable address (print char on write)

Priority: Medium — foundational for platform emulation features.