reorg: docs — library/device/ + library/protocol/ structure

Update the docs to match the reorganized tree:

- driver-model.md: the "Families" graph now shows library/device/<domain>/
  (data + logic split) and library/protocol/ instead of the old bus/ + proto/
  sketch; add the microkernel note (the kernel's one library/ import is the
  device-abi data module) and the rule that a driver's hardware command set is
  not a service-seam protocol.
- README.md "Repository layout": device-abi and the protocols move out of
  system/ into library/device/ and library/protocol/; rewrite the "public
  interface as a module" paragraph around the protocol tree and the device
  domains; fix the source-map paths.
- Sweep every remaining stale path reference across docs/ to the moved files.
- Fix vfs-protocol.zig's own header (the standalone VFS server retired; the fat
  server is the backend today).

zig build + test green.

docs/testing.md

@@ -9,7 +9,7 @@ There are two layers:
 
 - **Host unit tests** (`zig build test`) — for pure, platform-independent logic.
   What began as the three shared contracts (`system/boot-handoff.zig`,
-  `system/abi.zig`, `system/devices/device-abi.zig`) now spans ~26 modules:
+  `system/abi.zig`, `library/device/model/device-abi.zig`) now spans ~26 modules:
   protocol and on-wire definitions (VFS, USB, virtio-gpu), the FAT engine, the
   display compositor, PS/2 and HID decoding, the kernel log ring, and the
   runtime's `time`/`thread` — the full list is the test step in `build.zig`.