feat: Phase 8.3 async/await syntax + runtime

- AST: declFuncIsAsync flag, ekAwait expression kind
- Parser: async func declarations, expr.await postfix operator
- Sema: allow await anywhere (blocks current thread until task completes)
- HIR: ekAwait lowered to bux_task_join(handle) call
- C backend: async func emitted as regular function
- Example: examples/async.bux + examples_pkg/async project
- All 20 existing examples pass, selfhost build works

examples/async.bux

@@ -0,0 +1,23 @@
+import Std::Io::{PrintLine, PrintInt};
+import Std::Task::TaskHandle;
+
+async func BackgroundWork(id: int) {
+    PrintLine("Task started:");
+    PrintInt(id);
+    PrintLine("");
+    Task_Sleep(50);
+    PrintLine("Task done:");
+    PrintInt(id);
+    PrintLine("");
+}
+
+func Main() -> int {
+    PrintLine("Main: spawning tasks...");
+    let h1: *void = spawn BackgroundWork(1);
+    let h2: *void = spawn BackgroundWork(2);
+    PrintLine("Main: waiting for tasks...");
+    h1.await;
+    h2.await;
+    PrintLine("Main: all tasks completed");
+    return 0;
+}