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/kernel/process.zig

@@ -137,6 +137,7 @@ pub fn init() void {
     architecture.setSystemCallHandler(system_call);
     scheduler.terminate_current_hook = terminateCurrentLocked;
     scheduler.reap_task_hook = reapTaskLocked;
+    scheduler.timer_tick_hook = timerSweepLocked;
 }
 
 /// Return -1 (as an unsigned bit pattern) in the system_call result register.
@@ -202,6 +203,9 @@ fn system_call(state: *architecture.CpuState) void {
         .process_kill => systemProcessKill(state),
         .process_exit_reason => systemProcessExitReason(state),
         .process_subscribe => systemProcessSubscribe(state),
+        .signal_bind => systemSignalBind(state),
+        .process_signal => systemProcessSignal(state),
+        .timer_bind => systemTimerBind(state),
         _ => fail(state),
     }
 }
@@ -575,6 +579,20 @@ fn releaseTaskResourcesLocked(t: *scheduler.Task) void {
     recordExitLocked(t);
     irq.releaseOwner(t.id);
     devices_broker.releaseAllOwnedBy(t.id);
+    // The dying task's signal endpoint and one-shot timers go with it.
+    if (t.signal_endpoint) |raw| {
+        ipc.dropRef(@ptrCast(@alignCast(raw)));
+        t.signal_endpoint = null;
+    }
+    t.pending_signals = 0;
+    for (&one_shot_timers) |*slot| {
+        if (slot.*) |timer| {
+            if (timer.owner == t.id) {
+                ipc.dropRef(timer.endpoint);
+                slot.* = null;
+            }
+        }
+    }
     // A dead subscriber's own subscriptions go first: it must not hear about
     // itself, and the slots' endpoint references drop with it.
     for (&exit_subscribers) |*slot| {
@@ -735,6 +753,92 @@ fn systemProcessSubscribe(state: *architecture.CpuState) void {
     failErr(state, ipc.ENOSPC);
 }
 
+/// signal_bind(endpoint): nominate where this process's signals arrive — the
+/// IRQ-as-IPC pattern a fourth time (docs/process-lifecycle.md). Replacing a
+/// binding drops the old reference; signals that pended while unbound are
+/// delivered immediately on bind, coalesced into one notification.
+fn systemSignalBind(state: *architecture.CpuState) void {
+    const t = scheduler.current();
+    if (t.aspace == 0) return fail(state);
+    const endpoint = ipc.resolveHandle(t, architecture.systemCallArg(state, 0)) orelse return failErr(state, ipc.EBADF);
+    const flags = sync.enter();
+    defer sync.leave(flags);
+    if (t.signal_endpoint) |raw| ipc.dropRef(@ptrCast(@alignCast(raw)));
+    endpoint.refcount += 1;
+    t.signal_endpoint = @ptrCast(endpoint);
+    if (t.pending_signals != 0) {
+        ipc.notifyLocked(endpoint, abi.notify_signal_bit | t.pending_signals);
+        t.pending_signals = 0;
+    }
+    architecture.setSystemCallResult(state, 0);
+}
+
+/// process_signal(id, signal): post a signal — a one-way, coalescing statement,
+/// never a question (docs/process-lifecycle.md). The authority gate is the
+/// supervision link, like kill; a process may also signal itself. Unbound
+/// targets accumulate the signal in their pending mask.
+fn systemProcessSignal(state: *architecture.CpuState) void {
+    const t = scheduler.current();
+    if (t.aspace == 0) return fail(state);
+    const id = architecture.systemCallArg(state, 0);
+    const signal = architecture.systemCallArg(state, 1);
+    if (id > std.math.maxInt(u32)) return failErr(state, ipc.ESRCH);
+    if (signal > 31) return failErr(state, ipc.EBADF); // not a Signal bit position
+    const flags = sync.enter();
+    defer sync.leave(flags);
+    const target = scheduler.taskByIdLocked(@intCast(id)) orelse return failErr(state, ipc.ESRCH);
+    if (target.aspace == 0) return failErr(state, ipc.ESRCH);
+    if (target.supervisor != t.id and target.id != t.id) return failErr(state, ipc.EPERM);
+    target.pending_signals |= @as(u32, 1) << @intCast(signal);
+    if (target.signal_endpoint) |raw| {
+        const endpoint: *ipc.Endpoint = @ptrCast(@alignCast(raw));
+        ipc.notifyLocked(endpoint, abi.notify_signal_bit | target.pending_signals);
+        target.pending_signals = 0;
+    }
+    architecture.setSystemCallResult(state, 0);
+}
+
+/// The one-shot timers of timer_bind: the missing timed wait. A service arms a
+/// deadline and keeps serving; the expiry arrives in the same replyWait as
+/// everything else (notify_timer_bit). What stop-sequence escalation, hello
+/// deadlines, and restart backoff are built from — and later, `alarm`.
+const timer_capacity = 16;
+const OneShotTimer = struct { deadline: u64, endpoint: *ipc.Endpoint, owner: u32 };
+var one_shot_timers: [timer_capacity]?OneShotTimer = .{null} ** timer_capacity;
+
+/// Sweep expired timers — hung on scheduler.timer_tick_hook, so it runs on every
+/// tick with the big kernel lock held, like the sleeper wake it rides beside.
+fn timerSweepLocked() void {
+    const now = architecture.millis();
+    for (&one_shot_timers) |*slot| {
+        if (slot.*) |timer| {
+            if (now >= timer.deadline) {
+                ipc.notifyLocked(timer.endpoint, abi.notify_timer_bit);
+                ipc.dropRef(timer.endpoint);
+                slot.* = null;
+            }
+        }
+    }
+}
+
+/// timer_bind(endpoint, ms): arm a one-shot timer. -ENOSPC when the table is full.
+fn systemTimerBind(state: *architecture.CpuState) void {
+    const t = scheduler.current();
+    if (t.aspace == 0) return fail(state);
+    const endpoint = ipc.resolveHandle(t, architecture.systemCallArg(state, 0)) orelse return failErr(state, ipc.EBADF);
+    const ms = architecture.systemCallArg(state, 1);
+    const flags = sync.enter();
+    defer sync.leave(flags);
+    for (&one_shot_timers) |*slot| {
+        if (slot.* == null) {
+            endpoint.refcount += 1;
+            slot.* = .{ .deadline = architecture.millis() + ms, .endpoint = endpoint, .owner = t.id };
+            return architecture.setSystemCallResult(state, 0);
+        }
+    }
+    failErr(state, ipc.ENOSPC);
+}
+
 fn systemProcessExitReason(state: *architecture.CpuState) void {
     const t = scheduler.current();
     if (t.aspace == 0) return fail(state);