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/block-device/src/lib.rs

@@ -0,0 +1,21 @@
+#![no_std]
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct SectorId(u32);
+
+impl SectorId {
+    pub const fn new(id: u32) -> Self {
+        Self(id)
+    }
+
+    pub const fn raw(&self) -> u32 {
+        self.0
+    }
+}
+
+pub trait BlockDevice {
+    fn sector_size(&self) -> usize;
+    fn sector_count(&self) -> usize;
+    fn read_sector(&mut self, sector: SectorId, buf: &mut [u8]) -> bool;
+    fn write_sector(&mut self, sector: SectorId, buf: &[u8]) -> bool;
+}