init: /protocol replaces the ServiceId registry

A protocol is reached by name now, not by a compile-time integer. Init is
PID 1 and already knows which binary it started, so init serves /protocol
as a vfs backend: bind claims a contract with the provider's endpoint
attached, open answers with that endpoint as the reply's capability, and
readdir lists what is bound with the task and binary behind it. The kernel
reserves the prefix — nothing may mount over it, under it, or unmount it —
and ServiceId, ipc_register and ipc_lookup are gone, their syscall numbers
left vacant.

A bind is authorized by who the caller *is*: the kernel-stamped binary
together with the supervising task's identity, matched against
/system/configuration/protocol.csv. Identity, not spelling — spawn is
ungated, so an attacker can run any bundled binary, and a name-only rule
would have let it launder grants through an init of its own making. A name
a live process holds is refused to everyone else; a dead one's is released.

Three review rounds against a hostile ring-3 process found what 108 green
tests could not, because the suite contains no attacker. Publishing init's
supervision endpoint as the registry put PID 1's mailbox in every process's
hands, where two forged bytes reached the shutdown path: privileged traffic
is now believed only from the task that holds the contract it speaks for.
A capability arriving on a request outlived every path that ignored it,
one handle per call until the table was full — in init, and in the harness
ten services share — so the arriving capability is owned by the turn and
released unless a handler says otherwise. And the kernel let anyone holding
an endpoint handle aim signals, timers, exit notices and interrupts at it:
binding now requires having created it.

Suite 108/108. The new protocol-registry case asserts eleven properties,
each one an attack that must fail.

library/device/usb/usb.zig

@@ -16,6 +16,7 @@
 //! the service harness drops buffered-message payloads — see service.zig).
 
 const std = @import("std");
+const channel = @import("channel");
 const ipc = @import("ipc");
 const time = @import("time");
 const usb_transfer_protocol = @import("usb-transfer-protocol");
@@ -130,13 +131,15 @@ pub const Device = struct {
     }
 };
 
-/// Look up the USB bus and open the device with the assigned id, handing over a
-/// freshly created endpoint for asynchronous interrupt reports. Retries while the
-/// bus is still coming up (a class driver races the bus driver at boot).
+/// Open `/protocol/usb-transfer` and, on that channel, open the device with the
+/// assigned id, handing over a freshly created endpoint for asynchronous interrupt
+/// reports. Retries while the bus is still coming up (a class driver races the bus
+/// driver at boot). Two opens, deliberately: the first names the contract, the
+/// second names an object within it.
 pub fn open(device_id: u64) ?Device {
     var attempts: usize = 0;
     const bus = while (attempts < 100) : (attempts += 1) {
-        if (ipc.lookup(.usb_bus)) |handle| break handle;
+        if (channel.openEndpoint("usb-transfer")) |handle| break handle;
         time.sleepMillis(20);
     } else return null;