feat(hw): add XNOR-popcount CAM design with cocotb verification

Implement a multi-lane Content Addressable Memory (CAM) that scores
rows by XNOR popcount against a query hash and returns the top-1 match.

RTL modules:
- popcount: parallel group-based population count
- argmax_update: iterative best-match tracking with tie-break
- cam_core: parameterized scanning engine (NUM_ROWS/HASH_BITS/LANES)
  with optional SIM_NOISE and SIM_DEBUG ifdef guards
- cam_top: thin wrapper exposing cam_core ports

Verification:
- Python reference model (ref_model.py) for score-level golden comparison
- cocotb testbench (test_cam_basic.py) covering write/query/reset and
  external noise mask scenarios with score debug verification
- Noise sweep script (sweep_noise.py) measuring top-1 stability under
  configurable bit-flip rates
- Verilator-oriented Makefile with parameterizable compile options

hw/rtl/popcount.sv

@@ -0,0 +1,41 @@
+`timescale 1ns/1ps
+
+module popcount #(
+    parameter int WIDTH = 512,
+    parameter int GROUP = 8,
+    parameter int OUT_WIDTH = $clog2(WIDTH + 1)
+) (
+    input  logic [WIDTH-1:0] bits_i,
+    output logic [OUT_WIDTH-1:0] count_o
+);
+    localparam int NUM_GROUPS = (WIDTH + GROUP - 1) / GROUP;
+    localparam int GROUP_COUNT_WIDTH = $clog2(GROUP + 1);
+
+    logic [GROUP_COUNT_WIDTH-1:0] group_counts [NUM_GROUPS];
+
+    genvar g;
+    generate
+        for (g = 0; g < NUM_GROUPS; g++) begin : gen_group_popcount
+            always_comb begin
+                group_counts[g] = '0;
+                for (int b = 0; b < GROUP; b++) begin
+                    int idx;
+                    idx = g * GROUP + b;
+                    if (idx < WIDTH) begin
+                        group_counts[g] = group_counts[g] + bits_i[idx];
+                    end
+                end
+            end
+        end
+    endgenerate
+
+    // Reduction over small group counts. This is intentionally simple and
+    // synthesis-friendly enough for the first prototype. If timing fails,
+    // replace this block with a fully staged adder tree.
+    always_comb begin
+        count_o = '0;
+        for (int i = 0; i < NUM_GROUPS; i++) begin
+            count_o = count_o + group_counts[i];
+        end
+    end
+endmodule