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/client/build.zig

@@ -12,10 +12,15 @@ pub fn build(b: *std.Build) void {
 
     const ipc = kernel.module("ipc");
     const time = kernel.module("time");
+    // Every client reaches its service by name now: resolve `/protocol/<name>`,
+    // open it, and take the provider's endpoint out of the reply
+    // (docs/os-development/protocol-namespace.md).
+    const channel = kernel.module("channel");
 
     _ = b.addModule("display-client", .{
         .root_source_file = b.path("display/display-client.zig"),
         .imports = &.{
+            .{ .name = "channel", .module = channel },
             .{ .name = "ipc", .module = ipc },
             .{ .name = "time", .module = time },
             .{ .name = "display-protocol", .module = protocol.module("display-protocol") },
@@ -24,6 +29,7 @@ pub fn build(b: *std.Build) void {
     _ = b.addModule("input-client", .{
         .root_source_file = b.path("input/input-client.zig"),
         .imports = &.{
+            .{ .name = "channel", .module = channel },
             .{ .name = "ipc", .module = ipc },
             .{ .name = "time", .module = time },
             .{ .name = "input-protocol", .module = protocol.module("input-protocol") },

library/client/display/display-client.zig

@@ -1,9 +1,10 @@
 //! User-space display client: talk to the display service (query the mode, and — from D3
 //! — create layers, draw, and present) without hand-rolling the IPC. The `runtime.block`
-//! shape: a cached `.display` lookup with a boot-race retry, then extern-struct request/
+//! shape: a cached `/protocol/display` open with a boot-race retry, then extern-struct request/
 //! reply marshalling. See system/services/display/ and docs/display.md.
 
 const std = @import("std");
+const channel = @import("channel");
 const ipc = @import("ipc");
 const time = @import("time");
 const display_protocol = @import("display-protocol");
@@ -19,13 +20,13 @@ pub const Info = struct {
 /// The service endpoint, looked up once and cached.
 var handle: ?ipc.Handle = null;
 
-/// Look up the display service, retrying while it comes up (a client races its
-/// registration at boot). Returns the endpoint, or null if it never appears.
+/// Open `/protocol/display`, retrying while it comes up (a client races the
+/// service's bind at boot). Returns the endpoint, or null if it never appears.
 fn service() ?ipc.Handle {
     if (handle) |h| return h;
     var attempts: usize = 0;
     while (attempts < 100) : (attempts += 1) {
-        if (ipc.lookup(.display)) |h| {
+        if (channel.openEndpoint("display")) |h| {
             handle = h;
             return h;
         }

library/client/input/input-client.zig

@@ -22,7 +22,7 @@
 //!     }
 
 const std = @import("std");
-const abi = @import("abi");
+const channel = @import("channel");
 const ipc = @import("ipc");
 const time = @import("time");
 const input_protocol = @import("input-protocol");
@@ -44,13 +44,13 @@ pub const device_mouse = input_protocol.device_mouse;
 pub const device_joystick = input_protocol.device_joystick;
 pub const device_all = input_protocol.device_all;
 
-/// Look up the input service, retrying while it is still coming up. Both a subscriber and
-/// a source race the service's registration at boot, so both wait for it here rather than
-/// failing. Returns the service endpoint handle, or null if it never appears.
+/// Open `/protocol/input`, retrying while it is still coming up. Both a subscriber and
+/// a source race the service's bind at boot, so both wait for it here rather than
+/// failing. Returns the provider's endpoint handle, or null if it never appears.
 fn lookupService() ?ipc.Handle {
     var attempts: usize = 0;
     while (attempts < 100) : (attempts += 1) {
-        if (ipc.lookup(.input)) |handle| return handle;
+        if (channel.openEndpoint("input")) |handle| return handle;
         time.sleepMillis(50);
     }
     return null;