feat(EX): corex_fa2.py — third dlopen module from comp 168 AST chain

Log analysis from dockerrizhi.txt (07-23 Sub168 run) reveals THREE
corex modules, not two:

  1. corex_gdn.py — GatedDeltaNet fused kernel (already implemented)
  2. corex_moe.py — MoE routing + expert GEMM (already implemented)
  3. corex_fa2.py — FlashAttention2 dispatch (NEW)

corex_fa2.py handles 32/36 attention layers with three modes:
  :333 → FA2 packed prefill (B=2 Hq=4 Hkv=1 D=256 max_q=2048)
  :507 → FA2 paged chunked prefill (B=1 max_q=17 cache_blocks=2)
  :225 → FA2 paged decode (B=1 max_k=45455 partition=256)

Wraps ixformer.contrib.vllm_flash_attn + ixf_F.vllm_single_query_cached_kv_attention.
These .so files EXIST in the base image (libixattn.so).

Also: wired corex_fa2 import into qwen3_5.py + deploy script.

qwen3_6_scripts/qwen3_5.py

@@ -133,6 +133,15 @@ try:
 except ImportError:
     pass
 
+_corex_fa2_available = False
+_corex_fa2_module = None
+try:
+    from vllm.model_executor.models import corex_fa2 as _corex_fa2_module
+    _corex_fa2_available = True
+    logger.info("CoreX FA2 module found — fused attention kernels available")
+except ImportError:
+    pass
+
 # EX Engine: fused MoE topk_softmax CUDA kernel (xllm CUB-based)
 _ex_moe_topk_softmax = None
 _ex_moe_topk_available = False