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/acpi/acpi.zig

@@ -16,7 +16,7 @@ const aml = @import("aml");
 const acpi_ids = @import("acpi-ids");
 const device = runtime.device;
 const protocol = @import("device-manager-protocol");
-const power = runtime.power_protocol;
+const power = @import("power-protocol");
 /// AML opcode/prefix bytes by name (`zero_opcode`, `byte_prefix`, …) — so the `_HID`
 /// integer decode names the opcodes instead of bare 0x0A/0x0B/… (docs/coding-standards.md).
 const opcodes = aml.opcodes;

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

system/services/power/protocol.zig

@@ -1,68 +0,0 @@
-//! The power protocol (docs/power.md): system power's domain-named surface,
-//! registered under `ServiceId.power`. On x86 the acpi service serves it; on
-//! ARM a PSCI/mailbox service will register the same id — subscribers never
-//! learn which firmware they are on (docs/discovery.md — firmware neutrality).
-//! The vfs-protocol pattern: extern-struct messages, a version, reserved fields.
-
-/// The protocol version a client states nowhere yet — reserved for the day a
-/// handshake needs it; requests carry it so a mismatch can be refused loudly.
-pub const version: u16 = 1;
-
-pub const Operation = enum(u8) {
-    /// Subscribe to power events: the subscriber's endpoint rides as the
-    /// call's capability (the input/device-manager pattern); events arrive on
-    /// it as buffered messages carrying an `EventMessage`.
-    subscribe = 1,
-    /// Orderly shutdown's last step: enter S5. Accepted only from PID 1
-    /// (init) — the process that has already run the stop sequence over
-    /// everything else.
-    shutdown = 2,
-    /// The published event payload (never sent *to* the service).
-    event = 3,
-};
-
-/// What happened. The vocabulary is hardware-neutral: a lid is a lid whether
-/// ACPI or a PSCI mailbox reported it.
-pub const Event = enum(u8) {
-    power_button = 1,
-    lid = 2,
-    ac = 3,
-    battery = 4,
-    /// A device notification that maps to none of the named events — the
-    /// `code` and `hid` fields say which device and what code.
-    notify = 5,
-};
-
-pub const Subscribe = extern struct {
-    operation: u8 = @intFromEnum(Operation.subscribe),
-    reserved0: u8 = 0,
-    version: u16 = version,
-    reserved1: u32 = 0,
-};
-
-pub const Shutdown = extern struct {
-    operation: u8 = @intFromEnum(Operation.shutdown),
-    reserved0: u8 = 0,
-    version: u16 = version,
-    reserved1: u32 = 0,
-};
-
-/// A published event, as the buffered-message payload subscribers receive.
-pub const EventMessage = extern struct {
-    operation: u8 = @intFromEnum(Operation.event),
-    /// An Event value.
-    event: u8,
-    reserved0: u16 = 0,
-    /// The device notification code (Notify's second argument), or 0.
-    code: u32 = 0,
-    /// The notifying device's hardware id (EISA-decoded), or all zero.
-    hid: [8]u8 = .{0} ** 8,
-};
-
-pub const Reply = extern struct {
-    status: i32,
-    reserved: u32 = 0,
-};
-
-/// Upper bound on any message in this protocol — sizes endpoint buffers.
-pub const message_maximum = 64;