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/sema.nim

@@ -1000,6 +1000,11 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
     case expr.exprIntrinsic
     of ikLine, ikColumn: return makeInt()
     of ikFile, ikFunction, ikDate, ikTime, ikModule: return makeStr()
+  of ekSpawn:
+    discard sema.checkExpr(expr.exprSpawnCallee, scope)
+    for arg in expr.exprSpawnArgs:
+      discard sema.checkExpr(arg, scope)
+    return makePointer(makeVoid())
   of ekSpread:
     return sema.checkExpr(expr.exprSpreadOperand, scope)