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>

docs/ch401_fuller_static_frame_closeout.md

@@ -0,0 +1,32 @@
+# Ch401 — fuller 56-draw static frame
+
+Ch401 retains the accepted Ch400 chronological base, one correctly paired ABE
+draw, two later-scene anchors, and the proven 24-draw central cluster. The
+result is 56 authentic draws in 28 bounded epochs, within the existing 32-epoch
+host/testbench limit.
+
+## Simulation
+
+- 638,022 fragments fed
+- 321,817 fragments passed Z
+- 28 ordered drains
+- zero request drops, color FIFO overflows, or AXI BRESP errors
+- Z mismatches: 0 / 307,200
+- color mismatches: 0 / 128,998
+
+## Silicon
+
+No RTL changed after the accepted Ch400 RBF. The 28-epoch board run passed all
+texture CRC, runtime palette, staging, record-count, drain, and zero-drop gates
+and returned `rc=0`.
+
+- nonzero pixels: 128,998
+- bounds: `(36,10)..(599,399)`
+- board/simulation MEM SHA-256:
+  `0383cc739aa8e9b921489a3fc1ffca7ab2a48763cb57028d4a90e3ae9a34142f`
+- board/simulation PNG SHA-256:
+  `a4806d0ba39fcea1b904375f715d11f7a81d1364c78f201f40c1bb5906ac008b`
+
+Board and simulation are byte-exact over all 307,200 framebuffer words.
+Ch401 is accepted on silicon and remains displayed through the LPDDR HDMI
+scanout.