feat(CRITICAL): ix_bridge call chain + upstream xllm/ds_vllm sync

3 changes that close the MoE performance gap:

1. _custom_ops.py: Add ix_bridge as Priority 0 for topk_softmax
   - Before: tries our .cu kernel (fails) → PyTorch fallback (1-3 TPS)
   - After: tries ix_bridge → ixformer::infer::topk_softmax() → FAST
   - Call chain: _custom_ops.topk_softmax() → ix_bridge.topk_softmax()
     → ix_moe_bridge.so → ixformer::infer::topk_softmax()

2. Dockerfile: Add ix_moe_bridge.cpp precompile step
   - This was the missing link: code existed but was never compiled
   - Uses torch.utils.cpp_extension.load() to link against libixformer.so

3. upstream_ref sync from GitHub (cloned, not rewritten):
   - xLLM-AI/xllm: ILU kernels + CUDA MoE + GDN fp32 state mgmt
   - Deep-Spark/vllm: latest MoE kernel sources

ex_engine/precompile_ix_bridge.py

@@ -0,0 +1,107 @@
+#!/usr/bin/env python3
+"""
+Precompile ix_moe_bridge.cpp during Docker build.
+
+This bridges Python ↔ ixformer::infer C++ API (topk_softmax, group_gemm, etc).
+Source: upstream_ref/xllm/xllm/core/kernels/ilu/fused_moe.cpp call pattern
+"""
+import os
+import sys
+import glob
+import logging
+
+logging.basicConfig(level=logging.INFO)
+logger = logging.getLogger("precompile_ix_bridge")
+
+def find_ixformer_libs():
+    """Find libixformer.so and related libraries for linking."""
+    extra_ldflags = []
+    ixf_lib_dirs = set()
+    
+    try:
+        import ixformer
+        ixf_dir = os.path.dirname(ixformer.__file__)
+        for so in glob.glob(os.path.join(ixf_dir, "*.so")):
+            if "cpython" not in so:
+                extra_ldflags.append(so)
+                ixf_lib_dirs.add(os.path.dirname(so))
+        for so in glob.glob(os.path.join(ixf_dir, "_ixformer_torch*.so")):
+            if so not in extra_ldflags:
+                extra_ldflags.append(so)
+    except ImportError:
+        logger.warning("ixformer not installed")
+    
+    corex_lib = "/usr/local/corex/lib64"
+    if os.path.isdir(corex_lib):
+        for lib in ["libixformer.so", "libixattn.so", "libcublas.so"]:
+            p = os.path.join(corex_lib, lib)
+            if os.path.exists(p) and p not in extra_ldflags:
+                extra_ldflags.append(p)
+                ixf_lib_dirs.add(corex_lib)
+    
+    for d in ixf_lib_dirs:
+        extra_ldflags.append(f"-Wl,-rpath,{d}")
+    
+    return extra_ldflags
+
+def main():
+    # Find the .cpp source
+    search = [
+        "/workspace/ex_engine/csrc/ix_moe_bridge.cpp",
+        os.path.join(os.path.dirname(__file__), "csrc", "ix_moe_bridge.cpp"),
+    ]
+    
+    cpp_path = None
+    for p in search:
+        if os.path.isfile(p):
+            cpp_path = p
+            break
+    
+    if not cpp_path:
+        logger.error("ix_moe_bridge.cpp not found in: %s", search)
+        sys.exit(1)
+    
+    logger.info("Compiling ix_moe_bridge from %s", cpp_path)
+    
+    extra_ldflags = find_ixformer_libs()
+    logger.info("Link flags: %s", extra_ldflags)
+    
+    if not extra_ldflags:
+        logger.error("No ixformer libraries found — cannot compile bridge")
+        sys.exit(1)
+    
+    try:
+        from torch.utils.cpp_extension import load
+        mod = load(
+            name="ix_moe_bridge",
+            sources=[cpp_path],
+            extra_cflags=["-O2", "-std=c++17"],
+            extra_ldflags=extra_ldflags,
+            verbose=True,
+        )
+        fns = [x for x in dir(mod) if not x.startswith("_")]
+        logger.info("SUCCESS: ix_moe_bridge compiled with functions: %s", fns)
+    except Exception as e:
+        logger.error("FAILED to compile ix_moe_bridge: %s", e)
+        # Also try ix_full_bridge.cpp
+        full_path = cpp_path.replace("ix_moe_bridge", "ix_full_bridge")
+        if os.path.isfile(full_path):
+            logger.info("Trying ix_full_bridge.cpp instead...")
+            try:
+                mod = load(
+                    name="ix_full_bridge",
+                    sources=[full_path],
+                    extra_cflags=["-O2", "-std=c++17"],
+                    extra_ldflags=extra_ldflags,
+                    verbose=True,
+                )
+                fns = [x for x in dir(mod) if not x.startswith("_")]
+                logger.info("SUCCESS: ix_full_bridge compiled with functions: %s", fns)
+            except Exception as e2:
+                logger.error("FAILED ix_full_bridge too: %s", e2)
+                sys.exit(1)
+        else:
+            sys.exit(1)
+
+if __name__ == "__main__":
+    main()