eagle3: proper residual chain + stateful KV cache

Two fixes to bring EAGLE3 forward in line with vllm's llama_eagle3.py
reference:

1. Residual chain: previously the residual added into post_attention_layernorm
   was the token embedding (wrong). Reference uses _norm_after_residual:
     residual = fused_h (pre-norm)
     hidden_states = hidden_norm(fused_h)
   Then post_attention_layernorm is a fused add_rmsnorm(attn_out, residual),
   and the final norm is another add_rmsnorm(mlp_out, residual_after_attn).
   Neither residual carries the embedding — both carry fused_h forward.

2. KV cache: previously the attention was approximated as "output = V"
   because seq_len=1 (no cache), effectively giving EAGLE no history.
   Add a real per-Eagle3Head KV cache (1 layer × [1, num_kv_heads,
   max_seq_len, head_dim] BF16) that grows as we call step(). Use the
   existing decode_attention kernel with a fresh contiguous slice of the
   cache each step. reset() clears current_len for a new sequence.

Result on 10 prompts × 32 tokens (γ=1, no batched verify yet):
  matched=true across all prompts
  acceptance_rate = 20.0% (was 4.7% before residual fix, 1.3% originally)
    - Prompt 00 "The capital of France is": 60% (18/30) — best case
    - Other prompts: 10-25% — matches EAGLE paper's observation that
      structured/factual prompts get higher acceptance

Sanity check (check-eagle3) on Paris prompt now shows:
  EAGLE top-5 pairing A: "." / " is" / "," / " Paris" / ".\n"
  MATCH: EAGLE agrees with target on next token.

speedup_e2e still 0.95x because γ=1 does 1 target decode per token
regardless of acceptance. Real speedup requires γ≥2 with a single
batched target-verify covering all γ draft tokens; that's the next step.

crates/xserv-model/src/eagle3.rs

@@ -47,7 +47,14 @@ pub struct Eagle3Head {
     num_heads: usize,
     num_kv_heads: usize,
     head_dim: usize,
+    max_seq_len: usize,
     rope_cache: RopeCache,
+    // Stateful 1-layer KV cache: [1, num_kv_heads, max_seq_len, head_dim] BF16.
+    // We slice `..current_len` for attention. The head is tiny (~64 KB per
+    // 1000 tokens) so pre-allocating max_seq_len wastes negligible memory.
+    k_cache: Tensor,
+    v_cache: Tensor,
+    current_len: usize,
 }
 
 impl Eagle3Head {
@@ -100,6 +107,17 @@ impl Eagle3Head {
 
         let rope_cache = RopeCache::new(max_seq_len, head_dim, rope_theta);
 
+        let k_cache = Tensor::zeros(
+            &[1, num_kv_heads, max_seq_len, head_dim],
+            DType::BF16,
+            Device::Cuda(device),
+        );
+        let v_cache = Tensor::zeros(
+            &[1, num_kv_heads, max_seq_len, head_dim],
+            DType::BF16,
+            Device::Cuda(device),
+        );
+
         Self {
             fc_wt,
             hidden_norm,
@@ -119,10 +137,19 @@ impl Eagle3Head {
             num_heads,
             num_kv_heads,
             head_dim,
+            max_seq_len,
             rope_cache,
+            k_cache,
+            v_cache,
+            current_len: 0,
         }
     }
 
+    /// Reset the internal KV cache for a fresh sequence.
+    pub fn reset(&mut self) {
+        self.current_len = 0;
+    }
+
     /// One draft step: produce a token in target vocabulary space.
     ///
     /// - `target_hidden`: 3 tensors [1, hidden_size] from target hook layers
@@ -132,13 +159,19 @@ impl Eagle3Head {
     ///
     /// Returns (draft_token_in_target_vocab, draft_logits_tensor).
     pub fn step(
-        &self,
+        &mut self,
         target_hidden: &[Tensor; 3],
         embed_table: &Tensor,
         prev_token: u32,
         position: usize,
     ) -> (u32, Tensor) {
         let eps = 1e-6f32;
+        assert!(
+            self.current_len < self.max_seq_len,
+            "EAGLE KV cache overflow: {} >= {}",
+            self.current_len,
+            self.max_seq_len
+        );
 
         // 1. Fuse target hidden states: concat [h_low, h_mid, h_high] → fc
         let h_cat = concat_hidden(target_hidden);
@@ -147,15 +180,16 @@ impl Eagle3Head {
         // 2. Embed previous token (shared with target)
         let emb = embedding(embed_table, &[prev_token]); // [1, hidden]
 
-        // 3. Concat normalized: [norm(emb), norm(fused_h)] → [1, 2*hidden]
+        // 3. Norm both, concat, remember residual = fused_h (pre-norm).
+        let residual = fused_h.clone();
         let emb_normed = rmsnorm(&emb, &self.input_layernorm, eps);
         let h_normed = rmsnorm(&fused_h, &self.hidden_norm, eps);
-        let attn_in = concat_last_dim(&emb_normed, &h_normed); // [1, 8192]
+        let attn_in = concat_last_dim(&emb_normed, &h_normed); // [1, 2*hidden]
 
-        // 4. Self-attention (no KV cache for simplicity in v0 — single query)
-        let q = matmul_2d(&attn_in, &self.q_proj_wt); // [1, num_heads*head_dim]
-        let k = matmul_2d(&attn_in, &self.k_proj_wt); // [1, num_kv*head_dim]
-        let v = matmul_2d(&attn_in, &self.v_proj_wt); // [1, num_kv*head_dim]
+        // 4. Q/K/V projection then RoPE (position from caller).
+        let q = matmul_2d(&attn_in, &self.q_proj_wt);
+        let k = matmul_2d(&attn_in, &self.k_proj_wt);
+        let v = matmul_2d(&attn_in, &self.v_proj_wt);
 
         let q_3d = q.reshape(&[1, self.num_heads, self.head_dim]);
         let k_3d = k.reshape(&[1, self.num_kv_heads, self.head_dim]);
@@ -163,39 +197,58 @@ impl Eagle3Head {
         rope_inplace(&q_3d, &self.rope_cache, &positions);
         rope_inplace(&k_3d, &self.rope_cache, &positions);
 
-        // Single-token attention: Q·K^T / sqrt(d) → softmax → V
-        // With seq_len=1, attention is trivial: output = V (weight=1.0)
-        let attn_out = v.reshape(&[1, self.num_kv_heads, self.head_dim]);
-        let attn_out = if self.num_heads != self.num_kv_heads {
-            repeat_kv_for_single_token(&attn_out, self.num_heads / self.num_kv_heads)
-        } else {
-            attn_out
-        };
+        // 5. Append new K/V to the internal cache at slot `current_len`, then
+        // build a contiguous view [1, num_kv_heads, current_len+1, head_dim].
+        let v_3d = v.reshape(&[1, self.num_kv_heads, self.head_dim]);
+        self.append_to_kv_cache(&k_3d, &v_3d);
+        self.current_len += 1;
+        let kv_len = self.current_len;
+        let k_view = self.k_cache.narrow(2, 0, kv_len).contiguous();
+        let v_view = self.v_cache.narrow(2, 0, kv_len).contiguous();
+
+        // 6. Attention: q [1, num_q_heads, 1, head_dim] × k/v [1, num_kv_heads, kv_len, head_dim]
+        let q_4d = q_3d.reshape(&[1, self.num_heads, 1, self.head_dim]);
+        let attn_out = decode_attention(&q_4d, &k_view, &v_view);
+
+        // 7. Merge heads and o_proj.
         let attn_merged = attn_out.reshape(&[1, self.num_heads * self.head_dim]);
-        let attn_proj = matmul_2d(&attn_merged, &self.o_proj_wt); // [1, hidden]
+        let attn_proj = matmul_2d(&attn_merged, &self.o_proj_wt);
 
-        // Residual from embedding
-        let x = add(&attn_proj, &emb);
+        // 8. Post-attn fused add_rmsnorm.
+        let (mlp_in, residual) =
+            add_rmsnorm(&attn_proj, &residual, &self.post_attention_layernorm, eps);
 
-        // 5. MLP
-        let normed = rmsnorm(&x, &self.post_attention_layernorm, eps);
-        let gate = matmul_2d(&normed, &self.gate_proj_wt);
-        let up = matmul_2d(&normed, &self.up_proj_wt);
-        let mlp_out = silu_mul(&gate, &up);
-        let down = matmul_2d(&mlp_out, &self.down_proj_wt);
-        let x = add(&x, &down);
+        // 9. MLP.
+        let gate = matmul_2d(&mlp_in, &self.gate_proj_wt);
+        let up = matmul_2d(&mlp_in, &self.up_proj_wt);
+        let hidden = silu_mul(&gate, &up);
+        let down = matmul_2d(&hidden, &self.down_proj_wt);
 
-        // 6. Final norm + lm_head
-        let x = rmsnorm(&x, &self.norm, eps);
-        let logits = matmul_2d(&x, &self.lm_head_wt); // [1, 32000]
+        // 10. Final fused add_rmsnorm → lm_head.
+        let (x, _) = add_rmsnorm(&down, &residual, &self.norm, eps);
+        let logits = matmul_2d(&x, &self.lm_head_wt);
 
-        // 7. Argmax in draft vocab → map to target vocab
         let draft_id = argmax_bf16_single(&logits);
         let target_id = (draft_id as i64 + self.d2t[draft_id as usize]) as u32;
-
         (target_id, logits)
     }
 
+    /// Write new K/V rows (shape [1, num_kv_heads, head_dim]) at position
+    /// `current_len` inside the [1, num_kv_heads, max_seq_len, head_dim] cache.
+    fn append_to_kv_cache(&mut self, new_k: &Tensor, new_v: &Tensor) {
+        let head_bytes = self.head_dim * self.k_cache.dtype().size_bytes();
+        for h in 0..self.num_kv_heads {
+            for (cache, src) in [(&self.k_cache, new_k), (&self.v_cache, new_v)] {
+                let dst = unsafe {
+                    (cache.data_ptr() as *mut u8)
+                        .add(((h * self.max_seq_len) + self.current_len) * head_bytes)
+                };
+                let s = unsafe { (src.data_ptr() as *const u8).add(h * head_bytes) };
+                d2d(dst, s, head_bytes);
+            }
+        }
+    }
+
     /// Map a draft-vocab token id to the full target-vocab id via d2t.
     pub fn map_draft_to_target(&self, draft_id: u32) -> u32 {
         (draft_id as i64 + self.d2t[draft_id as usize]) as u32