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/ch377_audit_log.md

@@ -0,0 +1,14 @@
+# Ch377 Audit Log - Eighteen-Draw Second-Palette Residency
+
+- `zsrte18` adds the final later, on-screen CBP `508` candidate to Ch376, producing a seven-epoch
+  `fill, reuse, reuse, rebind, reuse, reuse, reuse` runtime-CLUT sequence at native 640x480.
+- Local depth-4096 integration passes with 532,162 accepted fragments, 436,333 Z-pass updates, seven fresh drains,
+  zero drops, exact Z `0/307200`, and exact color `0/132936`. Request-FIFO high-water is 2,090, 940, 1,757,
+  100, 81, 84, and 21.
+- On the DE25, both palette banks, both texture fills, all 956 expected records, and every drop gate pass.
+- Board and traced integration framebuffers are byte-identical at SHA-256
+  `9b49a9b56cb99296d09b12c535145b64dd8ae80b32f06e2a9934801567fe95bb`.
+- Replay-color is exact at `307200/307200`: 532,162 fragments, 436,333 Z-pass updates, 132,936 covered pixels,
+  and zero uncovered writes.
+
+Ch377 closes the eighteen-draw two-palette residency expansion on silicon without RTL, constraint, or fit changes.