fix(runtime): bridge加载失败安全降级到Tier1(ixformer.functions)

根因：ixformer.h声明的是ixformer::infer::*命名空间，但base image
实际导出的是ixformer_torch_ext::*(in _ixformer_torch.so)和
ixformer::functions::*(in libixformer.so)。命名空间不匹配导致
bridge .so虽然编译成功但import时undefined symbol。

当前方案：bridge加载失败时安全降级到Tier1(ixformer.functions)，
这是base image的Python binding，底层调用同样的CUDA kernel，性能一致。
Tier1已有silu_and_mul/rms_norm/rotary_embedding/paged_attention等。

后续迭代：重写ixformer.h用正确的ixformer_torch_ext namespace。

ex_engine/python/ix_unified.py

@@ -116,8 +116,14 @@ def _load_bridge():
                     _bridge = mod
                     logger.info("ix_unified_bridge loaded from %s", so_path)
                     return _bridge
+                except (ImportError, OSError, SystemError) as e:
+                    logger.warning("Bridge load failed (expected if ixformer "
+                                   "namespace mismatch): %s: %s",
+                                   os.path.basename(so_path), e)
+                    continue
                 except Exception as e:
-                    logger.warning("Failed to load %s: %s", so_path, e)
+                    logger.warning("Bridge load unexpected error: %s", e)
+                    continue
 
     logger.info("ix_unified_bridge.so not found, using fallback dispatch")
     return None