feat(stage-8d-4): dispatch chunked_gated_delta_rule_recurrence at prefill

run_delta_rule_cuda now picks between the per-token kernel and the
BT=64 chunked variant based on seq_len. Threshold = 64 matches mistralrs.
Prefill on Qwen3.6-27B (typical seq_len in the hundreds) drops from
one block-launch per token to one per 64-token chunk.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/arch/qwen3_5/linear_attn.rs

@@ -406,14 +406,29 @@ fn run_delta_rule_cuda(
     let g_bh = g.flatten(0, 1)?.contiguous()?;
     let beta_bh = beta.flatten(0, 1)?.contiguous()?;
     let mut state_bh = state.flatten(0, 1)?.contiguous()?;
-    let output_bh = crate::cuda::gdn::gated_delta_rule_recurrence_cuda(
+    // For long prefills, the chunked kernel (BT=64) processes a chunk
+    // of tokens at a time instead of one-by-one — same delta-rule math,
+    // far fewer block launches. Threshold matches mistralrs.
+    const CHUNK_THRESHOLD: usize = 64;
+    let output_bh = if seq_len >= CHUNK_THRESHOLD {
+        crate::cuda::gdn::chunked_gated_delta_rule_recurrence_cuda(
             &q_bh,
             &k_bh,
             &v_bh,
             &g_bh,
             &beta_bh,
             &mut state_bh,
-    )?;
+        )?
+    } else {
+        crate::cuda::gdn::gated_delta_rule_recurrence_cuda(
+            &q_bh,
+            &k_bh,
+            &v_bh,
+            &g_bh,
+            &beta_bh,
+            &mut state_bh,
+        )?
+    };
     let core_attn_out = output_bh.reshape((batch_size, num_heads, seq_len, head_v_dim))?;
     let new_state = state_bh.reshape((batch_size, num_heads, head_k_dim, head_v_dim))?;
     Ok((core_attn_out, new_state))