feat: multi-core via SO_REUSEPORT + Process.fork

- Each CPU core gets its own Kemal server instance with SO_REUSEPORT
- Bypasses Kemal.run to manually create HTTP::Server with reuse_port: true
- Properly wires up 404 error handler (setup_404 equivalent)
- Parent process manages worker lifecycle with signal forwarding
- All 18 validation tests passing

Author: BennyFranciscus

frameworks/kemal/src/server.cr

@@ -187,9 +187,47 @@ post "/upload" do |env|
 end
 
 # ---------------------------------------------------------------------------
-# Server config
+# Server config — multi-core via SO_REUSEPORT + fork
 # ---------------------------------------------------------------------------
 
 Kemal.config.port = 8080
 Kemal.config.env = "production"
-Kemal.run
+
+# Disable Kemal's default signal handlers and logging for workers
+Kemal.config.shutdown_message = false
+Kemal.config.logging = false
+
+worker_count = System.cpu_count.to_i
+worker_count = 1 if worker_count < 1
+
+def start_server
+  # Build Kemal's handler chain manually
+  Kemal.config.setup
+
+  # Register 404 handler (normally done by Kemal.run)
+  unless Kemal.config.error_handlers.has_key?(404)
+    error 404 do
+      render_404
+    end
+  end
+
+  handlers = Kemal.config.handlers
+
+  # Create server with SO_REUSEPORT
+  server = HTTP::Server.new(handlers)
+  server.bind_tcp("0.0.0.0", 8080, reuse_port: true)
+  server.listen
+end
+
+if worker_count > 1
+  pids = [] of Int64
+  worker_count.times do
+    child = Process.fork { start_server }
+    pids << child.pid
+  end
+  Signal::INT.trap { pids.each { |p| Process.signal(Signal::TERM, p) rescue nil }; exit }
+  Signal::TERM.trap { pids.each { |p| Process.signal(Signal::TERM, p) rescue nil }; exit }
+  sleep
+else
+  start_server
+end