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.

library/protocol/vfs/vfs-protocol.zig

@@ -8,7 +8,10 @@
 //! side is `runtime.fs` (library/runtime/fs.zig), which programs use directly.
 //!
 //! This is user-space only — the kernel knows nothing of files or paths; it only moves the bytes.
-//! Shared by library/runtime/fs.zig (client) and system/services/vfs/vfs.zig (server).
+//! Shared by library/runtime/fs.zig (the client) and the mount backends that serve it (today
+//! the fat server, system/services/fat/). The standalone user-space VFS server it was first
+//! written against has retired — path routing moved into the kernel (system/kernel/vfs.zig,
+//! fs_resolve) — but the protocol module outlived it.
 
 pub const Operation = enum(u32) {
     open, // open(path) -> node id