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/kernel/memory/memory.zig

@@ -38,6 +38,7 @@ pub const DmaRegion = dma.Region;
 pub const dma_coherent = dma.coherent;
 pub const dma_write_combining = dma.write_combining;
 pub const dma_below_4g = dma.below_4g;
+pub const dma_shareable = dma.shareable;
 pub const dmaAlloc = dma.alloc;
 pub const dmaFree = dma.free;