iommu: DMA-region capabilities — per-grant reachability, protocol flag-day

Replaces L2's interim DMA pool (every buffer reachable by every claimed
device) with true per-grant confinement: a device reaches only buffers
whose capability was delegated to its driver.

Kernel:
- DmaRegionObject (handle kind 2): a delegation token naming a dma_alloc'd
  region, passable across processes on the IPC cap slot like an endpoint
  or shared-memory object. Frames stay owned by the allocating address
  space (freed on dma_free/teardown as before); the token carries a `dead`
  flag so a stale downstream handle can no longer bind a freed region.
- dma_alloc gains the dma_shareable flag: it returns a capability handle
  in r8 and every region is tracked in a registry. A task's own regions
  auto-bind into the devices it claims (its rings just work); foreign
  buffers are bound explicitly.
- dma_bind / dma_unbind / handle_close syscalls (51-53). dma_bind maps a
  held region (or shared-memory) capability into a claimed device's domain;
  it is idempotent. handle_close reclaims a table slot (raised 16 -> 32).
- dma_free and task death unmap a region from every domain and invalidate
  BEFORE its frames return to the allocator — the stale-IOTLB use-after-
  free window, closed structurally.

Protocols (flag-day): block gains attach, usb-transfer gains dma_attach —
each carries a region capability on the cap slot. fat allocates its bounce
buffer shareable and attaches it; usb-storage allocates its transport
buffers shareable, attaches them to the controller, and forwards fat's
capability downstream; usb-xhci-bus binds and closes; virtio-gpu binds its
shared scanout surface. The physical addresses on the wire are unchanged
(identity IOVA), so no register-programming code moved.

Cross-process DMA (fat -> usb-storage -> xHC) now flows only through
delegated capabilities. iommu-usb-storage / iommu-usb-hid / iommu-fault
all green under per-grant enforcement; 104/104 overall (fail-open paths
unchanged).

library/device/block/block.zig

@@ -28,6 +28,18 @@ pub const Device = struct {
         return .{ .block_size = result.block_size, .block_count = result.block_count };
     }
 
+    /// Hand the block server a DMA-region capability (`handle` — from a `shareable`
+    /// dma_alloc) so it forwards it to the controller and the buffer's physical
+    /// addresses become reachable by the device. Call once per buffer before naming it
+    /// in `read`/`write`. Harmless success when no IOMMU is enforcing.
+    pub fn attach(self: Device, handle: ipc.Handle) bool {
+        var request = block_protocol.Request{ .operation = @intFromEnum(block_protocol.Operation.attach), .lba = 0, .count = 0, .physical = 0 };
+        var reply: [block_protocol.reply_size]u8 = undefined;
+        const result = ipc.callCap(self.endpoint, std.mem.asBytes(&request), &reply, handle) catch return false;
+        if (result.len < block_protocol.reply_size) return false;
+        return std.mem.bytesToValue(block_protocol.Reply, reply[0..block_protocol.reply_size]).status == 0;
+    }
+
     /// Read `count` blocks starting at `lba` into the DMA buffer at `physical`.
     pub fn read(self: Device, lba: u64, count: u32, physical: u64) bool {
         return self.transfer(.read, lba, count, physical);

library/device/driver/driver.zig

@@ -99,6 +99,19 @@ pub fn msiBind(device_id: u64, endpoint: usize) ?Msi {
     return .{ .address = rax, .data = @intCast(rdx) };
 }
 
+/// Map a delegated DMA-region (or shared-memory) capability into a claimed device's
+/// IOMMU domain, so the device may DMA to that buffer. The caller must own `device_id`
+/// and hold `handle` (received over IPC or from its own `dma.alloc(.. | shareable)`).
+/// Idempotent. Returns true on success (and trivially when no IOMMU is present).
+pub fn dmaBind(device_id: u64, handle: usize) bool {
+    return !failed(sc.systemCall2(.dma_bind, device_id, handle));
+}
+
+/// Unmap a previously `dmaBind`'d buffer from the device's domain.
+pub fn dmaUnbind(device_id: u64, handle: usize) bool {
+    return !failed(sc.systemCall2(.dma_unbind, device_id, handle));
+}
+
 /// Drain and log any pending IOMMU translation faults, returning the count seen. A
 /// diagnostic: a driver that suspects its device attempted an out-of-domain DMA (or a
 /// test proving enforcement) forces the hardware's fault records to the log now. Returns

library/device/usb/usb.zig

@@ -99,6 +99,19 @@ pub const Device = struct {
         return std.mem.bytesToValue(usb_transfer_protocol.InterruptSubscribeReply, reply[0..@sizeOf(usb_transfer_protocol.InterruptSubscribeReply)]).status == 0;
     }
 
+    /// Hand the controller a DMA-region capability (`handle` — from a `shareable`
+    /// dma_alloc, or forwarded from another process) so it binds that buffer into its
+    /// IOMMU domain. Must be called for every buffer whose physical address this device
+    /// will name in a `bulk` transfer, before the transfer. Harmless (and a no-op
+    /// success) when no IOMMU is enforcing. Returns false on failure.
+    pub fn attachDma(self: *Device, handle: ipc.Handle) bool {
+        var request = usb_transfer_protocol.DmaAttachRequest{ .device_token = self.token };
+        var reply: [@sizeOf(usb_transfer_protocol.DmaAttachReply)]u8 = undefined;
+        const result = ipc.callCap(self.bus, std.mem.asBytes(&request), &reply, handle) catch return false;
+        if (result.len < @sizeOf(usb_transfer_protocol.DmaAttachReply)) return false;
+        return std.mem.bytesToValue(usb_transfer_protocol.DmaAttachReply, reply[0..@sizeOf(usb_transfer_protocol.DmaAttachReply)]).status == 0;
+    }
+
     /// One bulk transfer (IN or OUT per `endpoint_address`'s direction bit) to or
     /// from the caller's own DMA buffer at `physical`. Returns the bytes moved.
     pub fn bulk(self: *Device, endpoint_address: u8, physical: u64, length: u32) ?u32 {