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/ports/framebuffer/src/lib.rs

@@ -0,0 +1,10 @@
+#![no_std]
+
+pub trait Framebuffer {
+    fn width(&self) -> u16;
+    fn height(&self) -> u16;
+    fn write_pixel(&mut self, x: u16, y: u16, color: u32);
+    fn fill_rect(&mut self, x: u16, y: u16, w: u16, h: u16, color: u32);
+    fn clear(&mut self, color: u32);
+    fn flush(&mut self);
+}