feat(serve): plumb Config.max_actors through to smarm init

Config gained a max_actors: Option<usize> (None = smarm's DEFAULT_MAX_ACTORS
of 16_384). serve_with now applies it to the smarm runtime config, so the
per-connection actor slab can be sized to the deployment's peak concurrent
connections. Since each connection is one actor, the slab was the hard cap
on concurrent connections (previously an un-raisable 16_384) regardless of
RAM/fds; slots are ~256 B so raising it is cheap next to per-conn stacks.

Verified on the GPU box: default caps at 16_383 held connections; with
max_actors raised, a paced ramp holds 100_000 concurrent slow-header
connections at 12.4 KB RSS / 2 VMAs each (1.29 GB total) on one pinned core.

src/serve.rs

@@ -65,6 +65,12 @@ pub struct Config {
     /// Default: 256 KiB. The reserve is virtual/demand-paged, so raising it
     /// costs address space, not RSS, until a handler actually uses it.
     pub conn_stack_reserve: usize,
+    /// Maximum concurrently-live actors — smarm's fixed slot slab, allocated
+    /// once at init. Each connection is one actor, so this is also the hard
+    /// cap on concurrent connections. `None` uses smarm's default (16_384).
+    /// Slots are ~256 B, so raising this is cheap relative to per-connection
+    /// stacks; size it to peak concurrent connections.
+    pub max_actors: Option<usize>,
 }
 
 /// Default per-connection actor stack reserve (see [`Config::conn_stack_reserve`]).
@@ -90,6 +96,7 @@ impl Config {
             max_message_bytes:  4 * 1024 * 1024,
             scheduler_threads:  None,
             conn_stack_reserve: DEFAULT_CONN_STACK_RESERVE,
+            max_actors:         None,
         }
     }
 
@@ -326,10 +333,13 @@ pub fn serve_with_shutdown(
     let conn_stack_reserve = config.conn_stack_reserve;
     let drain_timeout      = config.drain_timeout;
 
-    let smarm_cfg = match config.scheduler_threads {
+    let mut smarm_cfg = match config.scheduler_threads {
         Some(n) => smarm::Config::exact(n),
         None    => smarm::Config::default(),
     };
+    if let Some(m) = config.max_actors {
+        smarm_cfg = smarm_cfg.max_actors(m);
+    }
     let rt = smarm::init(smarm_cfg);
     // Listener self-termination flag — see the shutdown sequence below.
     let shutdown_flag = Arc::new(AtomicBool::new(false));