library: the last three protocols speak the envelope

These were the awkward ones. Each began with an operation packed into a
single byte — two of them with a version wedged in beside it — so there was
no wrapping them: the layouts had to be rebuilt. The device manager's own
enumerate and subscribe become the reserved verbs that mean the same thing
everywhere, its replies lose three status structs the envelope already
carries, and a device id becomes the packet's target. Power drops the
version it repeated on every request, because describe is the handshake,
and stops claiming a 64-byte ceiling it never needed for calls. USB moves a
control transfer's data to the packet tail in both directions, which makes
the status length the transferred length and retires a field that had been
saying the same thing twice.

The danger in this one was not the protocols but their readers. Init
recognised a power button by two bytes at the head of a message, the ACPI
service dispatched on the first byte, the xHCI driver read its operation
with a raw integer load, and the HID drivers reinterpreted a report
wholesale — none of which would have failed to compile once the layouts
moved. They would simply have stopped: no shutdown on the power button, no
reports from the keyboard. Every one of them now reads through the
generated types, and the shutdown gate that answers only a subscriber is
the same code it was.

Two sizes were decided by measuring rather than assuming. The child-added
message is both a request and the event broadcast to subscribers, and
alignment rounds it to 48 bytes, which puts its packet exactly on the
64-byte push floor — a test pins that, because a field added carelessly
would now overflow it. The interrupt report gives up eight bytes of inline
room to make space for the header; the two drivers that produce reports
send eight and four.

Suite 110/110.

library/device/driver/driver.zig

@@ -8,6 +8,7 @@ const abi = @import("abi");
 const device_abi = @import("device-abi");
 const sc = @import("system-call");
 const channel = @import("channel");
+const envelope = @import("envelope");
 const ipc = @import("ipc");
 const time = @import("time");
 const device_manager_protocol = @import("device-manager-protocol");
@@ -168,6 +169,9 @@ const lookup_pause_ms: u64 = 20;
 /// (best-effort standalone bring-up) or it refused the handshake. Bus drivers keep the handle
 /// to report children through; a driver that runs fine unsupervised discards it with `_ =`,
 /// and one that requires supervision bails on null. Logs the outcome itself.
+///
+/// The device this driver was assigned is the packet's `Header.target` — the manager's
+/// object addressing, so `no_device` here is a driver that serves none.
 pub fn hello(role: Role, device_id: u64) ?ipc.Handle {
     var attempts: u32 = 0;
     const manager = while (attempts < lookup_attempts) : (attempts += 1) {
@@ -178,15 +182,25 @@ pub fn hello(role: Role, device_id: u64) ?ipc.Handle {
         return null;
     };
 
-    const message = device_manager_protocol.Hello{ .role = @intFromEnum(role), .device_id = device_id };
-    var reply: [device_manager_protocol.reply_size]u8 = undefined;
-    const length = ipc.call(manager, std.mem.asBytes(&message), &reply) catch {
+    var packet: [device_manager_protocol.message_maximum]u8 = undefined;
+    const framed = device_manager_protocol.Protocol.encodeRequest(
+        .hello,
+        device_id,
+        .{ .role = @intFromEnum(role) },
+        &.{},
+        &packet,
+    ) orelse return null;
+
+    var reply: [device_manager_protocol.message_maximum]u8 = undefined;
+    const length = ipc.call(manager, framed, &reply) catch {
         std.log.info("hello call failed", .{});
         return null;
     };
-    if (length < device_manager_protocol.reply_size or
-        std.mem.bytesToValue(device_manager_protocol.HelloReply, reply[0..device_manager_protocol.reply_size]).status != 0)
-    {
+    const status = envelope.statusOf(reply[0..length]) orelse {
+        std.log.info("hello answered nothing readable", .{});
+        return null;
+    };
+    if (status.status != 0) {
         std.log.info("hello refused", .{});
         return null;
     }