[MMU] - Implement full DMG memory map routing

[Waterfountain10]

What to do:

Implement correct routing for:

0x0000–0x7FFF → Cartridge

0x8000–0x9FFF → VRAM

0xA000–0xBFFF → Cartridge RAM

0xC000–0xDFFF → WRAM

0xE000–0xFDFF → Echo RAM (mirror or ignore)

0xFE00–0xFE9F → OAM

0xFF00–0xFF7F → I/O registers

0xFF80–0xFFFE → HRAM

0xFFFF → IE register

Ensure unmapped regions return 0xFF.

Criteria:

CPU never directly accesses memory containers.

All reads/writes go through MMU.

Address routing matches Pan Docs DMG spec.