feat: Phase 8.3 Concurrency — tasks, channels, spawn

- Add async/await/spawn keywords to lexer
- C runtime: pthread wrappers (bux_task_spawn, bux_task_join, bux_task_sleep)
- C runtime: mutex/condvar channel implementation (bux_channel_new/send/recv/close/free)
- Build command adds -pthread flag for C compilation
- Std::Task module: TaskHandle, Task_Spawn, Task_Join, Task_Sleep
- Std::Channel module: generic Channel<T> with Send, Recv, Close, Free
- Parser: spawn Expr() expression
- Sema: ekSpawn returns *void
- HIR: hSpawn node lowered from ekSpawn
- C backend: emit bux_task_spawn(FuncName, arg) for spawn expressions
- Example: examples/concurrency.bux (thread + channel demo)
- Example: examples_pkg/concurrency working project

examples/concurrency.bux

@@ -0,0 +1,15 @@
+import Std::Io::{PrintLine, PrintInt};
+import Std::Task::TaskHandle;
+import Std::Channel::Channel;
+
+func Worker(arg: *void) -> *void {
+    PrintLine("Hello from thread!");
+    return null;
+}
+
+func Main() -> int {
+    let handle: *void = spawn Worker();
+    Task_Join(TaskHandle { handle: handle });
+    PrintLine("Thread finished");
+    return 0;
+}