Signals over IPC, one-shot timers, and the service harness (M17.4)

Signals are statements delivered as coalescing notifications to the
endpoint a process nominates with signal_bind — never a hijacked stack,
never a question (liveness is the zero-length ping the harness answers).
process_signal is supervisor-or-self gated, like kill; unbound targets
accumulate a pending mask delivered on bind. timer_bind is the missing
timed wait: a one-shot deadline landing in the same replyWait as
everything else — what stop(), hello deadlines, and restart backoff are
built from. runtime.service.run folds requests, signals, and
notifications into callbacks; the VFS conversion deletes its hand-rolled
loop and gains the whole lifecycle contract. The signals scenario drives
ping, reload, terminate->exited, the timer, and the deaf-child
deadline->killed path from ring 3. docs/process-lifecycle.md increments
1-4 are now as-built.

system/services/vfs/vfs.zig

@@ -138,42 +138,36 @@ fn handle(message: []const u8, out: []u8, sender: u32) usize {
     }
 }
 
-pub fn main() void {
-    const endpoint = runtime.ipc.createIpcEndpoint() orelse {
-        _ = runtime.system.write("vfs: no endpoint\n");
-        return;
-    };
-    if (!runtime.ipc.register(.vfs, endpoint)) {
-        _ = runtime.system.write("vfs: register failed\n");
-        return;
-    }
-    // First subscriber of the published exit events (docs/process-lifecycle.md):
-    // when a client dies holding open handles, the notification below is how the
-    // VFS learns to release them.
+/// Startup, under the harness: subscribe to the published exit events — when a
+/// client dies holding open handles, the exit notification is how the VFS learns
+/// to release them (docs/process-lifecycle.md).
+fn initialise(endpoint: runtime.ipc.Handle) bool {
     if (!runtime.process.subscribeExits(endpoint)) {
         _ = runtime.system.write("vfs: exit subscription failed\n");
     }
     _ = runtime.system.write("vfs: ready\n");
+    return true;
+}
 
-    var reply_buffer: [protocol.message_maximum]u8 = undefined;
-    var reply_len: usize = 0;
-    var receive: [protocol.message_maximum]u8 = undefined;
-    while (true) {
-        const got = runtime.ipc.replyWait(endpoint, reply_buffer[0..reply_len], &receive, null);
-        if (got.isChildExit()) {
-            // A published exit event: a process died; drop whatever it held.
-            releaseClientHandles(got.childProcessId());
-            reply_len = 0;
-            continue;
-        }
-        if (got.isNotification()) {
-            reply_len = 0; // not a request; nothing owed
-            continue;
-        }
-        reply_len = handle(receive[0..got.len], &reply_buffer, got.senderTaskId());
+/// A non-signal notification: the only kind the VFS subscribes to is exit events.
+fn onNotification(badge: u64) void {
+    if (badge & runtime.ipc.notify_exit_bit != 0) {
+        releaseClientHandles(@intCast(badge & ~(runtime.ipc.notify_badge_bit | runtime.ipc.notify_exit_bit)));
     }
 }
 
+pub fn main() void {
+    // The harness owns the loop: requests dispatch to handle(), exit events to
+    // onNotification(), ping and terminate are answered for free — this service
+    // gained the whole lifecycle contract by deleting its hand-rolled loop.
+    runtime.service.run(protocol.message_maximum, .{
+        .service = .vfs,
+        .init = initialise,
+        .on_message = handle,
+        .on_notification = onNotification,
+    });
+}
+
 pub const panic = runtime.panic;
 comptime {
     _ = &runtime.start._start;