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/integration/tb_ee_pad_buffer_branch.sv

@@ -249,6 +249,9 @@ module tb_ee_pad_buffer_branch;
     logic        feeder_ready_i   = 1'b0;
     logic [15:0] feeder_records_i = 16'd0;
     logic [31:0] feeder_waits_i   = 32'd0;
+    logic        frame_drained_i  = 1'b0;   // Ch353 — new bridge diagnostic input; tied off (.* matches by name)
+    logic        clear_done_i     = 1'b0;   // Ch357 — persistent-Z preclear ack; tied off (.* matches by name)
+    logic [31:0] frag_drops_i     = 32'd0;  // Ch357 — persistent-Z drop count; tied off (.* matches by name)
     ps2_hps_bridge u_bridge (
         .clk            (bclk),
         .reset_n        (breset_n),