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/ch386_audit_log.md

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+# Ch386 Audit Log - Frame-3 Eighteen-Draw Native Scene
+
+- Frame 1's remaining in-bounds PSMT8 candidates were rejected by the existing supported-state gates, so Ch386
+  starts a fresh, authentic frame-3 scene rather than weakening those gates.
+- `zsrtf3e18` contains 18 draws from `TBP=12288/CBP=14208`, partitioned into five ordered staging-safe epochs
+  (70, 106, 58, 97, and 192 records) with one texture fill and four CRC-verified reuses.
+- Local depth-4096 integration passes with 67,295 accepted fragments, 32,435 Z-pass updates, five fresh drains,
+  zero drops, exact Z `0/307200`, and exact color `0/20276`.
+- The DE25 completed every record, runtime-palette copy, texture residency check, and zero-drop gate. Board and
+  traced integration framebuffers are byte-identical at SHA-256
+  `8a905aa968f252fdc029832a5a66ee910c93053f791132c5ebf35ddbd4c08818`.
+- Replay-color is exact at `307200/307200`: 67,295 fragments, 32,435 Z-pass updates, 20,276 covered pixels, and
+  zero uncovered writes.
+
+Ch386 closes the first fresh frame-3 native scene on the Ch368 timing-closed core.