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/hir.nim

@@ -31,6 +31,8 @@ type
     hCast
     hIs
     hSizeOf
+    # Concurrency
+    hSpawn
     # Composite
     hBlock
     hStructInit
@@ -107,6 +109,9 @@ type
       isType*: Type
     of hSizeOf:
       sizeOfType*: Type
+    of hSpawn:
+      spawnCallee*: string
+      spawnArgs*: seq[HirNode]
     of hBlock:
       blockStmts*: seq[HirNode]
       blockExpr*: HirNode