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/ch425_postprocess_bisection_closeout.md

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+# Ch425 — post-process bisection closeout
+
+## Objective
+
+Append the captured post-process families to the accepted Ch424 scene and
+admit them only if the production replay stays exact and the PCSX2 fidelity
+score improves.
+
+## Candidates and isolation
+
+`zsrt139f20` concatenated the 125-epoch Ch424 scene, the 20-epoch PSMCT32
+darken pass, the 212-epoch Ch405 PSMT8 alpha-fan fixture, and the 18-epoch
+PSMT4 pass.  The 375-epoch production replay was functionally clean but its
+frame was catastrophically white (mapped MAE 165.23 / RMSE 189.17), so it was
+not deployed.
+
+Epoch-boundary replays isolated each family:
+
+- Ch424 + CT32 darken: mapped MAE 21.3646 / RMSE 30.1358, worse than the
+  Ch424 baseline 20.9469 / 29.7626.
+- Ch424 + CT32 + old alpha fans: mapped MAE 161.9749 / RMSE 185.4484; this is
+  the white-frame source.
+- Ch424 + PSMT4 only (`zsrt139f21`): mapped MAE 23.4472 / RMSE 32.3453 and
+  most scene detail was erased.
+
+Every isolation replay had zero fragment drops and exact RTL Z/color
+scoreboards.  The problem was therefore captured-state interpretation, not
+timing, loss, or board nondeterminism.  Ch425 rejected all three fixtures as
+generated and left the live board on Ch424 while the alpha source was audited.
+
+## Status
+
+Closed as a diagnostic chapter.  Ch426 supersedes the alpha result after
+repairing the fixture's RGBAQ alpha packing.