feat: Phase 8.3 true non-blocking async/await with stackful coroutines

- C runtime: ucontext-based coroutines (bux_async_spawn/yield/await/run)
- Round-robin scheduler in bux_async_run() manages ready queue
- spawn Func() without args → bux_async_spawn() creates coroutine
- await → blocks/yields until coroutine completes
- async func → emitted as regular C function, runs on dedicated stack
- Interleaved execution: multiple coroutines yield cooperatively
- Example: examples/async.bux demonstrates WorkA/WorkB interleaving
- All 20 examples pass, selfhost build works

src/c_backend.nim

@@ -253,14 +253,14 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
     return &"sizeof({typ})"
 
   of hSpawn:
-    var argsStr = ""
     if node.spawnArgs.len > 0:
-      # Package arguments into a heap-allocated struct (simplified: single arg)
+      # Fallback to OS thread spawn for functions with arguments
+      var argsStr = ""
       let arg = be.emitExpr(node.spawnArgs[0])
       argsStr = &"(void*){arg}"
+      return &"bux_task_spawn({node.spawnCallee}, {argsStr})"
     else:
-      argsStr = "NULL"
-    return &"bux_task_spawn({node.spawnCallee}, {argsStr})"
+      return &"bux_async_spawn({node.spawnCallee})"
 
   of hIf:
     # Ternary expression