feat: Phase 8.3 true non-blocking async/await with stackful coroutines

- C runtime: ucontext-based coroutines (bux_async_spawn/yield/await/run)
- Round-robin scheduler in bux_async_run() manages ready queue
- spawn Func() without args → bux_async_spawn() creates coroutine
- await → blocks/yields until coroutine completes
- async func → emitted as regular C function, runs on dedicated stack
- Interleaved execution: multiple coroutines yield cooperatively
- Example: examples/async.bux demonstrates WorkA/WorkB interleaving
- All 20 examples pass, selfhost build works

src/hir_lower.nim

@@ -770,7 +770,7 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
 
   of ekAwait:
     let lowered = ctx.lowerExpr(expr.exprAwaitOperand)
-    return hirCall("bux_task_join", @[lowered], makeVoid(), loc)
+    return hirCall("bux_async_await", @[lowered], makeVoid(), loc)
 
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),