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/README.md

@@ -245,10 +245,9 @@ system/          → /system   danos's own internals (the self-representation)
                               fat.zig, engine.zig, on-disk.zig)
 library/         → /lib       libraries, one sub-directory each
   runtime/                    the danos-native runtime + file API (fs) — the stable application ABI
-  mmio/                       volatile register access + memory barriers
-  device/                     device code by domain — model/ pci/ usb/ acpi/ — each a
+  device/                     device code by domain — mmio/ model/ pci/ usb/ acpi/ — each a
                               shareable data module (device-abi, pci-class, usb-abi/ids,
-                              acpi-ids) plus a logic module (pci, usb, aml)
+                              acpi-ids) plus a logic module (mmio, pci, usb, aml)
   protocol/                   driver↔service wire contracts (vfs block display scanout input
                               power device-manager usb-transfer), one module per directory
 boot/            → /boot      the loaders