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/display-server/Cargo.toml

@@ -3,17 +3,10 @@ name = "display-server"
 version = "0.1.0"
 edition = "2024"
 
-[[bin]]
-name = "display-server"
-test = false
-bench = false
-
-[features]
-default = ["qemu-virt"]
-qemu-virt = []
-
 [dependencies]
 types = { workspace = true }
-userspace = { workspace = true }
-userspace-rt = { workspace = true }
 server-framework = { workspace = true }
+port-framebuffer = { workspace = true }
+
+[dev-dependencies]
+driver-framebuffer-mock = { workspace = true }