perf(runtime): reject non-Live slots lock-free in the root-exit sweep

`shutdown_forest_roots` locked every slot's `cold` to discover `actor == None`.
The slab is `max_actors` entries (16_384 by default) and is almost entirely
vacant at root exit, so the sweep cost one uncontended mutex round-trip per
slot: a fixed ~152 µs per run on the 5900X, visible as deep_recursion 1T
289 -> 441 µs and the same +152 on chained_spawn 1T, fan_out_compute 1T and
deep_recursion 20T. `general.rs` times `init` and `run` together, so it landed
in every smarm bench number while no tokio section paid it.

`is_live_for` is a lock-free snapshot of the slot word; a non-Live slot has no
actor to shut down, and the ones that survive the check take the lock below as
before. The sweep is not weakened: the snapshot can go Live just after we pass
it, but that was already true of a spawn landing after the scan finished, and
there is no second sweep either way.

Suite green under reltest, including upstream's root_exit / shutdown /
supervisor_shutdown / root_sweep_trace (the last also under --features
smarm-trace, which is what asserts the sweep actually visits its targets).

src/runtime.rs

@@ -2056,6 +2056,22 @@ fn shutdown_forest_roots(inner: &Arc<RuntimeInner>, root: Pid) {
         if pid == root {
             continue;
         }
+        // Lock-free reject first. The slab is `max_actors` entries (16_384 by
+        // default) and is almost entirely vacant at root exit, so locking every
+        // slot's `cold` to discover `actor == None` made this scan cost one
+        // uncontended mutex round-trip per slot — a fixed ~150 µs per run on a
+        // 5900X, and `general.rs` times `init` + `run` together, so it landed
+        // in every smarm bench number. A non-Live slot has no actor to shut
+        // down, and the lock is taken again below for the ones that do.
+        //
+        // This does not weaken the sweep. `is_live_for` is a snapshot, so a
+        // slot can go Live just after we pass it — but that was already true
+        // of a spawn landing after the scan finished, and there is no second
+        // sweep either way (an actor that outlives this scan is its spawner's
+        // business, per the rule below).
+        if !slot.is_live_for(pid) {
+            continue;
+        }
         // Read the parent under the cold lock (generation-verified); act
         // outside it — `request_shutdown_inner` sends and may unpark.
         let parent = {