reorg: normalise every protocol alias to its module name

Finish the direct-import convention: a protocol is now aliased to its module
name in snake_case in every file that imports one — consumers and the runtime
client wrappers alike. This kills the last of the alias variety (the generic
`protocol`, plus `power` and `transfer`), so `device_manager_protocol.Hello`
means the same thing everywhere and grepping a module name finds all its uses.

Pure rename (identifier uses only; prose references left untouched via a
guarded pattern). 14 files, 299/299 lines.

zig build + test green; 16 QEMU cases pass (smoke, device-manager,
driver-restart, device-list, pci-scan, usb-hid, usb-storage, display-service,
display-native, display-reattach, input, acpi-ps2, vfs, fat-mount,
power-button, orderly-shutdown).

system/drivers/pci-bus/pci-bus.zig

@@ -12,7 +12,7 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
-const protocol = @import("device-manager-protocol");
+const device_manager_protocol = @import("device-manager-protocol");
 const device = runtime.device;
 const pci_class = @import("pci-class");
 
@@ -32,7 +32,7 @@ fn logFunction(bus: u64, dev: u64, function: u64, class_triple: u32) void {
     _ = runtime.system.write(text);
 }
 
-var bridge_id: u64 = protocol.no_device;
+var bridge_id: u64 = device_manager_protocol.no_device;
 var ecam_base: usize = 0;
 var ecam_physical: u64 = 0;
 var start_bus: u64 = 0;
@@ -209,13 +209,13 @@ fn registerAndReport(bus: u64, dev: u64, function: u64, class_triple: u32) void
         std.log.info("register refused for {d}:{d}.{d}", .{ bus, dev, function });
         return;
     };
-    const report = protocol.ChildAdded{
+    const report = device_manager_protocol.ChildAdded{
         .parent = bridge_id,
         .bus_address = (bus << 8) | (dev << 3) | function,
         .identity = class_triple,
         .device_id = registered,
     };
-    var reply: [protocol.message_maximum]u8 = undefined;
+    var reply: [device_manager_protocol.message_maximum]u8 = undefined;
     _ = runtime.ipc.call(manager_handle, std.mem.asBytes(&report), &reply) catch {
         std.log.info("child report for {d}:{d}.{d} failed", .{ bus, dev, function });
     };
@@ -235,7 +235,7 @@ pub fn main(init: runtime.process.Init) void {
         std.log.info("malformed bridge device id '{s}'", .{argument});
         return;
     };
-    runtime.service.run(protocol.message_maximum, .{
+    runtime.service.run(device_manager_protocol.message_maximum, .{
         .init = initialise,
         .on_message = onMessage,
     });