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/cam_top.sv

@@ -0,0 +1,64 @@
+`timescale 1ns/1ps
+
+module cam_top #(
+    parameter int NUM_ROWS  = 512,
+    parameter int HASH_BITS = 512,
+    parameter int LANES     = 16,
+
+    parameter int ROW_BITS   = $clog2(NUM_ROWS),
+    parameter int SCORE_BITS = $clog2(HASH_BITS + 1)
+) (
+    input  logic clk,
+    input  logic rst_n,
+
+    input  logic                  wr_en,
+    input  logic [ROW_BITS-1:0]   wr_row,
+    input  logic [HASH_BITS-1:0]  wr_hash,
+
+    input  logic                  query_valid,
+    output logic                  query_ready,
+    input  logic [HASH_BITS-1:0]  query_hash,
+
+    output logic                  result_valid,
+    input  logic                  result_ready,
+    output logic [ROW_BITS-1:0]   top1_index,
+    output logic [SCORE_BITS-1:0] top1_score,
+
+`ifdef SIM_NOISE
+    input  logic [LANES*HASH_BITS-1:0] noise_mask_lanes_flat,
+`endif
+
+`ifdef SIM_DEBUG
+    output logic [NUM_ROWS*SCORE_BITS-1:0] score_debug_flat,
+`endif
+
+    output logic busy
+);
+    cam_core #(
+        .NUM_ROWS(NUM_ROWS),
+        .HASH_BITS(HASH_BITS),
+        .LANES(LANES),
+        .ROW_BITS(ROW_BITS),
+        .SCORE_BITS(SCORE_BITS)
+    ) u_core (
+        .clk(clk),
+        .rst_n(rst_n),
+        .wr_row(wr_row),
+        .wr_hash(wr_hash),
+        .wr_en(wr_en),
+        .query_valid(query_valid),
+        .query_ready(query_ready),
+        .query_hash(query_hash),
+        .result_valid(result_valid),
+        .result_ready(result_ready),
+        .top1_index(top1_index),
+        .top1_score(top1_score),
+`ifdef SIM_NOISE
+        .noise_mask_lanes_flat(noise_mask_lanes_flat),
+`endif
+`ifdef SIM_DEBUG
+        .score_debug_flat(score_debug_flat),
+`endif
+        .busy(busy)
+    );
+endmodule