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/abi.zig

@@ -77,6 +77,9 @@ pub const SystemCall = enum(u64) {
     fs_mount = 48, // fs_mount(prefix_ptr, prefix_len, backend_handle, rewrite_ptr, rewrite_len) -> 0/-errno: mount a userspace filesystem's endpoint at an absolute prefix (possession of the handle is the capability)
     fs_unmount = 49, // fs_unmount(prefix_ptr, prefix_len) -> 0/-errno: remove a backend mount
     iommu_fault_drain = 50, // iommu_fault_drain() -> count: drain + log pending IOMMU translation faults (a diagnostic; the count of faults seen this call)
+    dma_bind = 51, // dma_bind(device_id, region_handle) -> 0/-errno: map a DMA-region capability into the claimed device's IOMMU domain (idempotent). The caller must own the device and hold the handle
+    dma_unbind = 52, // dma_unbind(device_id, region_handle) -> 0/-errno: unmap a previously bound region from the device's domain and invalidate
+    handle_close = 53, // handle_close(handle) -> 0/-errno: drop one capability handle and free its table slot (endpoints, shared-memory, DMA regions)
     _,
 };
 
@@ -114,6 +117,7 @@ pub const msi_address_base: u64 = 0xFEE0_0000;
 pub const dma_coherent: u64 = 1; // strong-uncacheable — the default, the only portable one
 pub const dma_write_combining: u64 = 2; // write-combining (framebuffers); needs PAT
 pub const dma_below_4g: u64 = 4; // physical address must fit 32 bits (legacy DMA engines)
+pub const dma_shareable: u64 = 8; // return a capability handle (r8) so the region can be delegated + dma_bound
 
 /// Set in the badge returned by `ipc_reply_wait` when what arrived is an
 /// **asynchronous notification** (a device interrupt bound with `irq_bind`, or a