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_lower.nim

@@ -756,6 +756,18 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
     return HirNode(kind: hLit, litToken: Token(kind: tkStringLiteral, text: "\"\"", loc: loc),
                    typ: makeStr(), loc: loc)
 
+  of ekSpawn:
+    var calleeName = ""
+    if expr.exprSpawnCallee.kind == ekIdent:
+      calleeName = expr.exprSpawnCallee.exprIdent
+    elif expr.exprSpawnCallee.kind == ekPath:
+      calleeName = expr.exprSpawnCallee.exprPath.join("_")
+    var args: seq[HirNode] = @[]
+    for arg in expr.exprSpawnArgs:
+      args.add(ctx.lowerExpr(arg))
+    return HirNode(kind: hSpawn, spawnCallee: calleeName, spawnArgs: args,
+                   typ: makePointer(makeVoid()), loc: loc)
+
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),
                    typ: makeVoid(), loc: loc)