feat: add live statistics with per-trial snapshots and von Neumann option (FPGA v5)

fpga/rtl/entropy_generator.v

@@ -15,6 +15,7 @@ module entropy_generator #(
     input  wire [3:0] temporal,     // consecutive bits XORed per output bit, 1-8
     input  wire [7:0] ring_select,  // ring exposed on the characterization outputs
     output reg        out_bit,      // processed output, held for `temporal` clocks
+    output reg        out_valid,    // pulse: out_bit has just been updated
     output wire       spatial_bit,  // XOR of all rings, one per clock
     output wire [RINGS-1:0] ring_samples,  // synchronized enhanced outputs, for health tests
     output reg        ring_enhanced,
@@ -72,6 +73,7 @@ module entropy_generator #(
     wire      last = (phase >= temporal - 4'd1) || (temporal == 4'd0);
 
     always @(posedge clk) begin
+        out_valid <= last;
         if (last) begin
             out_bit     <= accumulator ^ spatial_bit;
             accumulator <= 1'b0;