Pipeline matmul multiplier: 2-stage to fix WNS

Split compute into stage1 (compute → partial) and
stage2 (partial → c_data). Halves CARRY4 chain depth
from 16 to ~8 levels. Compute cycles: 16 → 32.

rtl/ip/matmul/matmul_axi_slave.v

@@ -71,6 +71,8 @@ reg        busy;
 reg        done;
 reg        error;
 reg [3:0]  calc_index;
+reg [65:0] partial;       // pipeline register for 2-stage multiplier
+reg        pipe_valid;     // partial has valid data
 
 wire       write_fire;
 wire       start_fire;
@@ -288,17 +290,28 @@ always @(posedge clk) begin
             done       <= 1'b0;
             error      <= 1'b0;
             calc_index <= 4'd0;
+            pipe_valid <= 1'b0;
         end else if (busy) begin
-            c_data[calc_index] <= calc_c_element(calc_index);
+            if (!pipe_valid) begin
+                // Stage 1: compute element, store in partial
+                partial    <= calc_c_element(calc_index);
+                pipe_valid <= 1'b1;
+            end else begin
+                // Stage 2: store partial into c_data, advance
+                c_data[calc_index] <= partial;
+                pipe_valid <= 1'b0;
                 if (calc_index == 4'd15) begin
                     busy       <= 1'b0;
                     done       <= 1'b1;
         calc_index    <= 4'd0;
+        partial       <= 66'b0;
+        pipe_valid    <= 1'b0;
                 end else begin
                     calc_index <= calc_index + 4'd1;
                 end
             end
         end
+    end
 end
 
 endmodule