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/storage-server/src/lib.rs

@@ -1,7 +1,6 @@
 #![no_std]
 
 pub mod adapters;
-pub mod block_device;
 pub mod filesystem;
 pub mod fs;
 pub mod protocol;
@@ -30,7 +29,7 @@ fn debug_print(s: &[u8]) {
 pub fn run() -> ! {
     debug_print(b"storage: started\n");
 
-    let device = adapters::virtio_blk::VirtioBlockDevice::new();
+    let device = driver_block_virtio::VirtioBlockDevice::new();
     let fat16 = fs::fat16::Fat16::mount(device);
     if fat16.is_none() {
         debug_print(b"storage: FAT16 mount failed\n");
@@ -71,7 +70,7 @@ pub fn run() -> ! {
     }
 }
 
-fn read_entire_file<B: block_device::BlockDevice>(
+fn read_entire_file<B: port_block_device::BlockDevice>(
     fat: &mut fs::fat16::Fat16<B>,
     filename: &[u8],
     out: &mut [u8],