fix: fail-fast on ix_bridge failure + probe script for real machine

1. ix_bridge.py: RuntimeError instead of silent PyTorch fallback
   If JIT compile fails, crash immediately with diagnostic message.
   0 score with no error log is worse than a visible crash.

2. qwen3_5.py: explicit WARNING log on import failure (not silent)
   Shows exact error so we can diagnose from docker log.

3. probe_ixformer_symbols.py: definitive test for real machine
   - Finds all ixformer .so files
   - nm/objdump for topk_softmax C++ symbol
   - Checks Python bindings
   - Attempts JIT compile + link (the real test)
   - Prints PASS/FAIL with next-step instructions

Run on real machine: python3 probe_ixformer_symbols.py

qwen3_6_scripts/qwen3_5.py

@@ -74,6 +74,7 @@ except ImportError:
 # ix_bridge: C++ bridge to ixformer::infer::topk_softmax (bypasses missing Python binding)
 _ix_bridge_module = None
 _ix_bridge_available = False
+_ix_topk_softmax = None
 try:
     from ex_engine.python.ix_bridge import topk_softmax as _ix_topk_softmax
     _ix_bridge_available = True
@@ -88,8 +89,11 @@ except ImportError:
         from ex_engine.python.ix_bridge import topk_softmax as _ix_topk_softmax
         _ix_bridge_available = True
         logger.info("ix_bridge: ixformer C++ topk_softmax available (deployed path)")
-    except ImportError:
-        logger.info("ix_bridge: not available, MoE uses PyTorch topk")
+    except ImportError as e:
+        # NOT silent: log the exact error so we can diagnose from docker logs
+        logger.warning(
+            "ix_bridge: IMPORT FAILED (%s). MoE will use PyTorch topk. "
+            "This is 3x slower. Run probe_ixformer_symbols.py to diagnose.", e)
 _corex_gdn_available = False
 _corex_moe_available = False