runtime: make fatal failures unrecoverable

Match the Go runtime by terminating for deadlocks, stack overflows,
runtime and GC invariants, invalid lock operations, and platform
initialization failures instead of routing them through panic/recover.

Keep language-level runtime errors and unsupported user operations
recoverable. Add crash coverage that verifies fatal errors bypass deferred
recover calls.

src/runtime/scheduler_cooperative.go

@@ -51,7 +51,7 @@ var (
 func deadlock() {
 	// call yield without requesting a wakeup
 	task.Pause()
-	panic("unreachable")
+	runtimeFatal("unreachable")
 }
 
 func goexit() {
@@ -78,7 +78,7 @@ func addSleepTask(t *task.Task, duration timeUnit) {
 	if schedulerDebug {
 		println("  set sleep:", t, duration)
 		if t.Next != nil {
-			panic("runtime: addSleepTask: expected next task to be nil")
+			runtimeFatal("runtime: addSleepTask: expected next task to be nil")
 		}
 	}
 	now := ticks()