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

src/c_backend.nim

@@ -252,6 +252,16 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
     let typ = typeToC(node.sizeOfType)
     return &"sizeof({typ})"
 
+  of hSpawn:
+    var argsStr = ""
+    if node.spawnArgs.len > 0:
+      # Package arguments into a heap-allocated struct (simplified: single arg)
+      let arg = be.emitExpr(node.spawnArgs[0])
+      argsStr = &"(void*){arg}"
+    else:
+      argsStr = "NULL"
+    return &"bux_task_spawn({node.spawnCallee}, {argsStr})"
+
   of hIf:
     # Ternary expression
     let cond = be.emitExpr(node.ifCond)