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/bins/display-qemu/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "display-qemu"
+version = "0.1.0"
+edition = "2024"
+
+[[bin]]
+name = "display-qemu"
+test = false
+bench = false
+
+[dependencies]
+userspace-rt = { workspace = true }
+userspace = { workspace = true }
+server-framework = { workspace = true }
+display-server = { workspace = true }
+driver-framebuffer-virtio = { workspace = true }

crates/bins/display-qemu/src/main.rs

@@ -0,0 +1,14 @@
+#![no_std]
+#![no_main]
+
+extern crate userspace_rt;
+
+use driver_framebuffer_virtio::VirtioFramebuffer;
+
+#[unsafe(no_mangle)]
+extern "C" fn main() {
+    let fb = VirtioFramebuffer::new();
+    let mut srv = display_server::server::DisplayServer::new(fb);
+    server_framework::serve(&mut srv, 0);
+    loop { userspace::syscall::yield_now(); }
+}