feat: wire grounded selector into AR slot processing

将 Task 5 的 grounded selector 织入 AR 出题路径（Phase 3.5，位于
record_item 与 postprocess 之间），仅在 strategy.uses_grounded_selector
为真时进入。observation 始终落库（含 hard-fail），硬失败走重出。
PipelineConfig 新增 candidate_pool_size/selector_delta_low/
selector_delta_high 三参，YAML 与 CLI seed override 同步。

tools/generate_questions.py

@@ -912,6 +912,9 @@ async def _run_generate_v2(args: argparse.Namespace) -> None:
             concurrency=config.concurrency,
             seed=args.seed,
             output_dir=config.output_dir,
+            candidate_pool_size=config.candidate_pool_size,
+            selector_delta_low=config.selector_delta_low,
+            selector_delta_high=config.selector_delta_high,
         )
         logger.info("seed 覆盖为: {}", args.seed)