cfar_ca: reset detect_count per frame (AUDIT-C6)

Bug: 16-bit detect_count was reset only on power-on; increments at three
sites (ST_IDLE/ST_BUFFER simple-threshold paths and ST_CFAR_CMP) accumulate
across frames. At 178 fps with even 2-3 average detections per frame the
counter wraps in 100-180 seconds, breaking any rate-based host telemetry
or health check that reads it.

Fix: add `detect_count <= 16'd0` in ST_DONE so the counter represents
"detections this frame" instead of cumulative-since-boot. Updated $display
wording from "total detections" to "frame detections".

T13 flipped from "count keeps growing" to "identical-scene frames produce
identical counts" (the actual contract a per-frame counter must satisfy).
TB snapshots detect_count during ST_DONE because cfar_busy only goes low
on ST_IDLE entry — after the reset has fired.

Verification: tb_cfar_ca 24/24 PASS, quick regression 31/31 PASS.

Note: detect_count output port is now "live" (accumulates during frame,
0 between frames). Audit confirmed no current host telemetry consumes
this port. If future host code needs a stable last-frame total, add a
detect_count_last_frame snapshot register then.

9_Firmware/9_2_FPGA/cfar_ca.v

@@ -575,13 +575,21 @@ always @(posedge clk or negedge reset_n) begin
         // ================================================================
         // ST_DONE: Frame complete, return to idle
         // ================================================================
+        // AUDIT-C6 fix: reset detect_count per-frame so it represents
+        // "detections this frame" instead of "total since power-on". The
+        // 16-bit counter saturates after ~6500 frames at typical detection
+        // rates (tens of seconds of real traffic), breaking any rate-based
+        // host telemetry that reads it.
+        // ================================================================
         ST_DONE: begin
             cfar_status <= 8'd0;
             state <= ST_IDLE;
 
             `ifdef SIMULATION
-            $display("[CFAR] Frame complete: %0d total detections", detect_count);
+            $display("[CFAR] Frame complete: %0d frame detections", detect_count);
             `endif
+
+            detect_count <= 16'd0;
         end
 
         default: state <= ST_IDLE;

9_Firmware/9_2_FPGA/tb/tb_cfar_ca.v

@@ -19,7 +19,8 @@
  *   T10: Zero guard, zero train corner case
  *   T11: Reset during processing → clean recovery
  *   T12: Back-to-back frames → second frame processes correctly
- *   T13: detect_count accumulates across frames
+ *   T13: detect_count resets per frame (AUDIT-C6 fix); identical-scene frames
+ *        produce identical counts instead of cumulative-since-boot accumulation
  *   T14: cfar_busy asserts during processing, deasserts after
  */
 
@@ -63,6 +64,16 @@ wire [15:0] detect_count;
 wire        cfar_busy;
 wire [7:0]  cfar_status;
 
+// AUDIT-C6: detect_count is reset in ST_DONE before transitioning to
+// ST_IDLE, so reads after wait_cfar_done (which returns when cfar_busy
+// goes low at ST_IDLE) see 0. Snapshot the count whenever the FSM is in
+// ST_DONE so T13 can verify per-frame counting.
+reg [15:0] tb_detect_count_at_done;
+always @(posedge clk) begin
+    if (dut.state == 4'd7 /* ST_DONE */)
+        tb_detect_count_at_done <= dut.detect_count;
+end
+
 // ============================================================================
 // TEST TRACKING
 // ============================================================================
@@ -604,7 +615,8 @@ initial begin
     check(12, "T12: Back-to-back frame 2: target at (20,10) detected", find_detection(6'd20, 5'd10) == 1);
 
     // ================================================================
-    // T13: detect_count accumulates
+    // T13: detect_count per-frame reset (AUDIT-C6)
+    // Identical-scene frames must produce identical counts (not cumulative).
     // ================================================================
     test_num = 13;
     do_reset;
@@ -623,16 +635,24 @@ initial begin
     wait_cfar_done(20000);
     begin : t13_save
         reg [15:0] count_after_frame1;
-        count_after_frame1 = detect_count;
-        $display("  [INFO] T13: detect_count after frame 1 = %0d", count_after_frame1);
+        // Use TB snapshot of detect_count at ST_DONE (live counter is reset
+        // before ST_IDLE so direct read returns 0). See tb_detect_count_at_done.
+        count_after_frame1 = tb_detect_count_at_done;
+        $display("  [INFO] T13: detect_count at frame 1 ST_DONE = %0d", count_after_frame1);
 
-        // Frame 2 (same target)
+        // Sanity: frame 1 produced at least one detection
+        check(13, "T13: frame 1 produced detections", count_after_frame1 > 16'd0);
+
+        // Frame 2 (same target — should produce same per-frame count)
         feed_frame(16'd1000);
         pulse_frame_complete;
         wait_cfar_done(20000);
-        $display("  [INFO] T13: detect_count after frame 2 = %0d", detect_count);
+        $display("  [INFO] T13: detect_count at frame 2 ST_DONE = %0d", tb_detect_count_at_done);
 
-        check(13, "T13: detect_count increases after second frame", detect_count > count_after_frame1);
+        // AUDIT-C6 fix: per-frame counter, identical scene → identical count.
+        // Pre-fix this would have shown ~2x count_after_frame1 (cumulative).
+        check(13, "T13: detect_count resets per frame (==frame 1 count)",
+              tb_detect_count_at_done == count_after_frame1);
     end
 
     // ================================================================