refactor: extract shared hw-virtio crate, eliminate VirtIO protocol duplication

Two VirtIO block and two VirtIO GPU implementations (adapter + driver)
shared 90% of their protocol logic. Extracted the common code into
hw-virtio, parameterized by MmioAccess trait.

New crate: hw-virtio (no_std, zero deps)
  - mmio.rs: MmioAccess trait, DirectMmio, init_legacy_device(), scan
  - queue.rs: VirtioQueue with descriptor chain submit + poll
  - block.rs: block read protocol (3-descriptor chain)
  - gpu.rs: GPU commands (create_2d, attach_backing, set_scanout,
            transfer_to_host, resource_flush)

Adapter thin wrappers (identity-mapped, statics for DMA):
  - virtio_blk.rs: 220 → 53 lines
  - virtio_gpu.rs: 165 → 37 lines

Driver thin wrappers (mapped VAs, phys-info for DMA):
  - drivers/block/virtio: 139 → 70 lines
  - drivers/framebuffer/virtio: 146 → 95 lines

Also deleted input-server/input_device.rs (1-line re-export,
superseded by port-input-device).

crates/adapter-riscv/src/devices/storage_setup.rs

@@ -1,5 +1,4 @@
 use crate::hw::uart::Uart;
-use crate::hw::virtio_blk::{mmio_read, mmio_write};
 use crate::{kernel_ref, mmu};
 use types::id::ProcessId;
 
@@ -86,22 +85,8 @@ fn write_phys_info(pt: &mut mmu::PageTable, reqhdr_phys: usize) {
 }
 
 fn reinit_device(base: usize, vqueue_phys: usize) {
-    mmio_write(base, 0x070, 0);
-    while mmio_read(base, 0x070) != 0 {}
-
-    mmio_write(base, 0x070, 1);
-    mmio_write(base, 0x070, mmio_read(base, 0x070) | 2);
-    mmio_write(base, 0x028, 4096);
-    mmio_write(base, 0x014, 0);
-    mmio_write(base, 0x024, 0);
-    mmio_write(base, 0x020, 0);
-
-    mmio_write(base, 0x030, 0);
-    let queue_max = mmio_read(base, 0x034);
-    mmio_write(base, 0x038, 8u32.min(queue_max));
-    mmio_write(base, 0x040, (vqueue_phys / 4096) as u32);
-
-    mmio_write(base, 0x070, mmio_read(base, 0x070) | 4);
+    let mmio = hw_virtio::mmio::DirectMmio::new(base);
+    hw_virtio::mmio::init_legacy_device(&mmio, vqueue_phys, hw_virtio::queue::QUEUE_SIZE as u32);
 }
 
 fn map_shared_buffer(init_pt: &mut mmu::PageTable, storage_pt: &mut mmu::PageTable) {