Fix Blackwell: skip FlashMLA assertion + force CuTeDSL kernel

1. DeepseekV4MLAAttention.__init__ had a hard assertion that the
   attention backend MUST be FlashMLA. On Blackwell, FlashMLA doesn't
   work but we bypass it via _attention_impl_blackwell(). Added
   _is_blackwell flag to skip FlashMLA-specific init (fp8_ds_mla
   cache format conversion).

2. Added VLLM_NVFP4_GEMM_BACKEND=cutedsl env var to docker-compose.yml
   to force CuTeDSL kernel selection for NVFP4 linear layers.

3. Updated register_cutedsl_kernel.py to also register CuTeDSL in
   _NVFP4_BACKEND_TO_KERNEL dict (for the env var override path).

docker-compose.yml

@@ -11,6 +11,7 @@ services:
       - PYTHONUNBUFFERED=1
       - VLLM_RPC_TIMEOUT_MS=600000
       - CLAWMINE_DEBUG=1
+      - VLLM_NVFP4_GEMM_BACKEND=cutedsl
     command:
       - /model
       - --trust-remote-code