feat: add Gated DeltaNet linear-attention layer for Qwen3-5 (#1059)

Standalone GDN stack depthwise causal conv1d into a gated delta-rule
recurrence wired up for the Qwen3-5 checkpoint layout. Decoupled from
the full model assembly so it can land independently.
- `MergedColumnParallelLinear` (srt/layers/linear.py)
- GDN kernels + backend (srt/layers/attention/linear/)

Author: hlFu

python/sgl_jax/srt/kernels/gdn/__init__.py

@@ -0,0 +1,25 @@
+"""Gated DeltaNet (GDN) reference kernels.
+
+Public entry points:
+
+* :func:`ragged_gated_delta_rule_ref` — token-by-token ``lax.scan`` over a
+  packed ragged batch (extend / chunked-prefill).
+* :func:`decode_gated_delta_rule_ref` — parallel single-step recurrence
+  across the batch (decode fast path).
+* :func:`jax_causal_conv1d_prefill` / :func:`jax_causal_conv1d_update` —
+  depthwise causal conv1d helpers (ragged prefill + single-token decode).
+"""
+
+from sgl_jax.srt.kernels.gdn.gated_delta import (
+    decode_gated_delta_rule_ref,
+    jax_causal_conv1d_prefill,
+    jax_causal_conv1d_update,
+    ragged_gated_delta_rule_ref,
+)
+
+__all__ = [
+    "decode_gated_delta_rule_ref",
+    "jax_causal_conv1d_prefill",
+    "jax_causal_conv1d_update",
+    "ragged_gated_delta_rule_ref",
+]