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

stdlib/Std/Channel.bux

@@ -0,0 +1,35 @@
+module Std::Channel {
+
+extern func bux_channel_new(capacity: int64, elem_size: int64) -> *void;
+extern func bux_channel_send(handle: *void, elem: *void);
+extern func bux_channel_recv(handle: *void, out: *void) -> int;
+extern func bux_channel_close(handle: *void);
+extern func bux_channel_free(handle: *void);
+
+struct Channel<T> {
+    handle: *void;
+}
+
+func Channel_New<T>(capacity: int64) -> Channel<T> {
+    return Channel<T> { handle: bux_channel_new(capacity, sizeof(T)) };
+}
+
+func Channel_Send<T>(ch: *Channel<T>, value: T) {
+    bux_channel_send(ch.handle, &value);
+}
+
+func Channel_Recv<T>(ch: *Channel<T>) -> T {
+    var result: T;
+    bux_channel_recv(ch.handle, &result);
+    return result;
+}
+
+func Channel_Close<T>(ch: *Channel<T>) {
+    bux_channel_close(ch.handle);
+}
+
+func Channel_Free<T>(ch: *Channel<T>) {
+    bux_channel_free(ch.handle);
+}
+
+}