Snapshot: fog implementation + fidelity tooling baseline (pre bilinear-clamp fix)

Per-vertex GS fog end-to-end (gs_stub emit incl. persp_emit5, gs_prim_list_feeder
XYZ2->XYZF2 on PRIM.FGE, gs_make_sh3_scheduler_fixture.py F/FGE packing), new fog
TBs, fidelity attribution tooling. Functional baseline before removing the dead
bilinear lerp8 clamps (Codex: 161-node comb loop -> -0.042ns setup fail).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sim/tb/top/tb_top_psmct32_runtime_clut.sv

@@ -0,0 +1,60 @@
+// Integration proof for the runtime CLUT writer mux in the BRAM board top.
+`timescale 1ns/1ps
+
+module tb_top_psmct32_runtime_clut;
+    logic clk = 1'b0;
+    always #5 clk = ~clk;
+    logic rst_n, core_go;
+    logic runtime_wr_en, runtime_busy;
+    logic [7:0] runtime_wr_idx;
+    logic [31:0] runtime_wr_data;
+    logic [7:0] r,g,b;
+    logic hsync,vsync,de,core_halt,dma_done_seen,frame_seen,raster_overflow,frame_toggle,dma_done_toggle;
+    int errors;
+
+    top_psmct32_raster_demo_bram #(.PSMCT32_SWIZZLE(1'b0)) dut (
+        .clk, .rst_n, .core_go, .r, .g, .b, .hsync, .vsync, .de,
+        .core_halt, .dma_done_seen, .frame_seen, .raster_overflow,
+        .frame_toggle, .dma_done_toggle,
+        .joy_a_pressed_i(1'b0), .joy_b_pressed_i(1'b0),
+        .runtime_clut_wr_en_i(runtime_wr_en),
+        .runtime_clut_wr_idx_i(runtime_wr_idx),
+        .runtime_clut_wr_data_i(runtime_wr_data),
+        .runtime_clut_busy_i(runtime_busy)
+    );
+
+    task automatic runtime_write(input logic [7:0] idx, input logic [31:0] data);
+        @(posedge clk);
+        runtime_wr_idx  <= idx;
+        runtime_wr_data <= data;
+        runtime_wr_en   <= 1'b1;
+        @(posedge clk);
+        runtime_wr_en   <= 1'b0;
+    endtask
+
+    task automatic check(input string label, input logic [31:0] got, input logic [31:0] want);
+        if (got !== want) begin
+            $error("[%s] got 0x%08x expected 0x%08x",label,got,want);
+            errors = errors + 1;
+        end
+    endtask
+
+    initial begin
+        rst_n=0; core_go=0; runtime_wr_en=0; runtime_wr_idx=0; runtime_wr_data=0; runtime_busy=0; errors=0;
+        repeat(4) @(posedge clk);
+        rst_n=1;
+        runtime_write(8'h00,32'h10203040);
+        runtime_write(8'h37,32'hA1B2C3D4);
+        runtime_write(8'hFF,32'h55667788);
+        repeat(2) @(posedge clk);
+        check("clut[0]",dut.u_clut.mem[0],32'h10203040);
+        check("clut[55]",dut.u_clut.mem[8'h37],32'hA1B2C3D4);
+        check("clut[255]",dut.u_clut.mem[8'hFF],32'h55667788);
+        runtime_busy=1;
+        repeat(2) @(posedge clk);
+        if (dut.clut_busy !== 1'b1) begin $error("runtime busy did not stall CLUT consumer"); errors=errors+1; end
+        if(errors) $fatal(1,"tb_top_psmct32_runtime_clut FAILED: %0d errors",errors);
+        $display("tb_top_psmct32_runtime_clut PASS: writer mux + busy gate");
+        $finish;
+    end
+endmodule