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.

docs/os-development/threading.md

@@ -270,13 +270,15 @@ stays single-threaded and lean.
   - *Handles do not cross threads.* The handle table lives on the `Task`
     ([scheduler.zig](../../system/kernel/scheduler.zig)), so a handle number is meaningful
     only to the thread that created it — thread A's endpoint handle `3` is not thread B's.
-    A thread that needs to reach an endpoint another thread owns looks it up
-    (`ipc.lookup(service)`) to install its **own** handle to the same underlying endpoint.
-    This is how the display's mouse-listener thread reaches the compositor loop's endpoint
-    to poke it awake (docs/display.md).
+    A thread that needs to reach an endpoint another thread owns opens the name
+    (`channel.openEndpoint("display")`) to install its **own** handle to the same
+    underlying endpoint — an ordinary client open, with no special mechanism for the
+    fact that the provider happens to be this process. This is how the display's
+    mouse-listener thread reaches the compositor loop's endpoint to poke it awake
+    (docs/display.md).
   - *IPC syscalls that touch shared kernel state now serialize under the big kernel lock.*
-    `create_ipc_endpoint`/`ipc_register`/`ipc_lookup` allocate from the kernel heap and
-    mutate the global service registry, endpoint refcounts, and handle tables. Those paths
+    `create_ipc_endpoint` allocates from the kernel heap and
+    mutates endpoint refcounts and handle tables. Those paths
     were unlocked because a single-threaded process could not race itself; a multi-threaded
     one can, from two cores at once. They now take `sync.enter()` like `call`/`reply_wait`/
     `send` already did — the kernel heap has no lock of its own (heap.zig: "every kernel