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).

system/services/fat/fat.zig

@@ -118,7 +118,17 @@ fn tryBringUp() void {
         _ = logging.write("/system/services/fat: block geometry unavailable\n");
         return;
     };
-    const bounce = memory.dmaAlloc(engine.max_transfer_sectors * 512, memory.dma_coherent) orelse return;
+    // Shareable so the buffer's capability can be attached down the chain (block server
+    // -> controller), making its physical addresses reachable by the device under an
+    // enforcing IOMMU. No-op binding otherwise.
+    const bounce = memory.dmaAlloc(engine.max_transfer_sectors * 512, memory.dma_coherent | memory.dma_shareable) orelse return;
+    if (bounce.handle) |handle| {
+        if (!device.attach(handle)) {
+            _ = logging.write("/system/services/fat: could not attach the DMA bounce buffer\n");
+            return;
+        }
+        _ = ipc.close(handle); // the binding holds its own reference now
+    }
     ipc_block = .{ .device = device, .bounce = bounce };
 
     const block_device = engine.BlockDevice{