reorg: move mmio into library/device and spell out its API

mmio is device-driver code, so it joins the other domains under
library/device/mmio/ (module name "mmio" unchanged — a pure relocation, only
the build paths move). And its abbreviated function names are spelled out per
docs/coding-standards.md:

  read  -> readRegister          mb  -> memoryBarrier
  write -> writeRegister         rmb -> readMemoryBarrier
                                 wmb -> writeMemoryBarrier

All call sites updated (virtio-gpu, usb-xhci-library, pci.Function); the two
display-driver placeholders import mmio but use nothing, so they're untouched.
Docs (driver-model graph, README layout, drivers.md, the FHS note) follow the
new path and names.

zig build + test green; virtio-gpu, display-native, display-reattach, usb-hid,
usb-hub, usb-storage, pci-scan pass.

docs/drivers.md

@@ -290,8 +290,8 @@ Several things this list used to warn about are now available (see
 [driver-model.md](driver-model.md)): **port I/O** (`io_read`/`io_write`, claim-gated by
 the device's `io_port` resource — direct ring-3 `in`/`out` is still a #GP, so a PS/2 or
 16550 driver goes through these), **DMA memory** (`dma_alloc`: contiguous, pinned,
-uncacheable, physical address exposed), **memory barriers** (`library/mmio`'s
-`mb`/`rmb`/`wmb`, imported as the `mmio` module), **fault isolation** (a ring-3 fault kills only the faulting
+uncacheable, physical address exposed), **memory barriers** (`library/device/mmio`'s
+`memoryBarrier`/`readMemoryBarrier`/`writeMemoryBarrier`, imported as the `mmio` module), **fault isolation** (a ring-3 fault kills only the faulting
 process — `killCurrentProcess` — and the machine keeps running,
 [resilience](resilience.md)), and **reclaim + restart on death** (every path out of a
 process releases its claims and IRQ/MSI bindings — `releaseAllOwnedBy`,