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/ast.nim

@@ -117,6 +117,7 @@ type
     ekIs
     ekTry
     ekUnwrap        ## expr! — unwrap or panic
+    ekSpawn         ## spawn expr — create a new task
     ekBlock
     ekMatch
 
@@ -196,6 +197,9 @@ type
       exprTryType*: TypeExpr  # nil for Result?, or explicit target type
     of ekUnwrap:
       exprUnwrapOperand*: Expr
+    of ekSpawn:
+      exprSpawnCallee*: Expr
+      exprSpawnArgs*: seq[Expr]
     of ekBlock:
       exprBlock*: Block
     of ekMatch: