reorg: move power-protocol to library/protocol; finish the protocol tree

power-protocol -> library/protocol/power/power-protocol.zig. Its two consumers
(init and the acpi discovery service) import it directly; the
runtime.power_protocol re-export and the runtime module import are dropped (no
runtime client speaks it). The now-empty system/services/power/ is removed.

This completes library/protocol/: every driver<->service wire contract lives
there and is imported by module name, no protocol is re-exported through
runtime, and the alias chaos (dp/sp, mixed protocol/<x>_protocol) is resolved.
virtio-gpu-protocol stays a driver-private relative import (a hardware command
set, not a driver<->service seam — like virtio-pci.zig beside it).

zig build + test green; power-button, orderly-shutdown pass.

system/services/init/init.zig

@@ -19,7 +19,7 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
-const power = runtime.power_protocol;
+const power = @import("power-protocol");
 const build_options = @import("build_options");
 
 /// The system services init brings up at boot, in order, by binary path. This is