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.

Makefile

@@ -29,7 +29,10 @@ kani-kernel:
 kani: kani-types kani-domain kani-kernel
 
 build:
-	cargo build -p adapter-riscv --target $(RISCV_TARGET) --release
+	cargo build --target $(RISCV_TARGET) --release \
+		-p adapter-riscv -p init -p echo -p snake \
+		-p display-qemu -p storage-qemu \
+		-p input-server -p log-server -p permission-manager -p session-server
 
 image: build
 	scripts/make-disk-image.sh