refactor: extract ports/drivers/bins layers — clean dependency inversion

Separate abstractions from implementations following DIP:

  servers/              drivers/
      │                     │
      ▼                     ▼
    ports/  (trait-only crates, zero deps)

New crates:

ports/ (abstractions):
  - port-framebuffer: Framebuffer trait
  - port-block-device: BlockDevice trait + SectorId
  - port-input-device: InputDevice trait + InputEvent types

drivers/ (implementations depend on ports/):
  - driver-framebuffer-virtio: VirtIO GPU framebuffer
  - driver-framebuffer-mock: test double
  - driver-block-virtio: VirtIO block device

bins/ (composition roots wire server + driver):
  - display-qemu: DisplayServer<VirtioFramebuffer>
  - storage-qemu: storage server with VirtIO block

Servers no longer own their hardware traits or driver code.
Neither server nor driver depends on the other — both point
inward to the port abstraction.

crates/servers/input-server/src/adapters/mock.rs

@@ -1,5 +1,4 @@
-use crate::input_device::InputDevice;
-use crate::protocol::InputEvent;
+use crate::input_device::{InputDevice, InputEvent};
 
 pub struct MockInputDevice {
     events: [Option<InputEvent>; 16],

crates/servers/input-server/src/adapters/qemu_uart.rs

@@ -1,5 +1,4 @@
-use crate::input_device::InputDevice;
-use crate::protocol::{InputEvent, Key};
+use crate::input_device::{InputDevice, InputEvent, Key};
 
 const UART_BASE: usize = 0x10000000;
 const UART_LSR: usize = 5;