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/drivers/framebuffer/mock/src/lib.rs

@@ -0,0 +1,63 @@
+#![no_std]
+
+use port_framebuffer::Framebuffer;
+
+pub struct MockFramebuffer {
+    width: u16,
+    height: u16,
+    pixels: [(u16, u16, u32); 4096],
+    pixel_count: usize,
+    flushed: bool,
+}
+
+impl MockFramebuffer {
+    pub fn new(width: u16, height: u16) -> Self {
+        Self {
+            width,
+            height,
+            pixels: [(0, 0, 0); 4096],
+            pixel_count: 0,
+            flushed: false,
+        }
+    }
+
+    pub fn pixel_count(&self) -> usize {
+        self.pixel_count
+    }
+
+    pub fn was_flushed(&self) -> bool {
+        self.flushed
+    }
+
+    pub fn pixel_at(&self, index: usize) -> (u16, u16, u32) {
+        self.pixels[index]
+    }
+}
+
+impl Framebuffer for MockFramebuffer {
+    fn width(&self) -> u16 { self.width }
+    fn height(&self) -> u16 { self.height }
+
+    fn write_pixel(&mut self, x: u16, y: u16, color: u32) {
+        if self.pixel_count < self.pixels.len() {
+            self.pixels[self.pixel_count] = (x, y, color);
+            self.pixel_count += 1;
+        }
+    }
+
+    fn fill_rect(&mut self, x: u16, y: u16, w: u16, h: u16, color: u32) {
+        for dy in 0..h {
+            for dx in 0..w {
+                self.write_pixel(x + dx, y + dy, color);
+            }
+        }
+    }
+
+    fn clear(&mut self, _color: u32) {
+        self.pixel_count = 0;
+    }
+
+    fn flush(&mut self) {
+        self.flushed = true;
+    }
+}