Decode PS/2 keyboard input correctly

[pbalduino]

Goal

Return real key events from the PS/2 driver instead of filtering everything out.

Context

ps2kb_read currently ignores every scancode except the 0xFA ACK byte, meaning the driver never forwards actual key presses. Higher-level input therefore relies on the fallback port-polling getchar.

Definition of Done

• Parse make/break scancodes and translate them using keyboard_map.
• Buffer key events so ps2kb_read returns characters without busy waiting.
• Track modifier keys (shift/control) or document the limitations.
• Demonstrate typing via the driver (e.g., exercising the console).