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/ch424_full_population_subpixel_repair.md

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+# Ch424 — restore the full scene population under native 12.4 coverage
+
+## Root cause
+
+Ch416 was described as a one-variable native-subpixel conversion of Ch415,
+but its fixture was regenerated from only 113 draw IDs.  Ch415 actually fed
+766 captured draws grouped into 254 compatible state runs.  The 113 IDs were
+only the surviving groups' representative indices, so treating them as the
+source draw list silently discarded most of the scene.  The resulting
+software reference already contained the long spike/band geometry later seen
+bit-exactly in simulation and on the board; this was not an FPGA timing fault.
+
+The `--legacy-strip-kicks` compatibility switch also ignored captured ADC
+kick suppression.  Ch424 uses the parsed kick semantics and does not enable
+that switch.
+
+## Fixture repair
+
+`sh3_zsrt139f19` starts from the audited 766-draw / 254-group plan in
+`/tmp/sh3_opaque_qclip_plan.json`, retains authentic chronological grouping,
+and enables the already-proven production feature set:
+
+- authentic Z, color modulation, zero-color fog fold, and bilinear reference;
+- native GS XYZ 12.4 coordinates and pixel-center coverage;
+- authentic scissor clipping and normalized full-region clamp;
+- runtime CLUT staging and 14,500-fragment capacity epochs;
+- parsed ADC/kick behavior, with no legacy strip override.
+
+The repaired fixture schedules 125 epochs.  Its software reference restores
+the coherent booth, floor, lamp, character, fence, and bench geometry.  With
+the accepted Ch418 scanout mapping, comparison against the PCSX2 224139 frame
+is MAE 18.5560 / RMSE 26.2740.  The raw 640x480 buffer is MAE 21.0802 / RMSE
+32.3348.
+
+## Acceptance gates
+
+- Production-gradient full RTL replay must finish all 125 epochs with zero
+  drops and exact final Z/color comparisons.
+- The unchanged, timing-clean Ch423 RBF must pass the same fixture on hardware.
+- The board framebuffer must match the production-gradient RTL framebuffer
+  bit for bit before Ch424 becomes the visible baseline.
+
+## Production RTL acceptance
+
+The production registered-gradient Verilator replay completed all 125 epochs:
+157,208 fragments were fed, 138,825 passed Z, fragment drops remained zero,
+and the final comparisons reported 0/307,200 Z mismatches and 0/114,550 color
+mismatches.  Marker/drain counts were complete, `col_ovf=0`,
+`bresp_err=0`, and the testbench finished PASS.
+
+The raw RTL frame measures MAE 22.9740 / RMSE 33.6203 against PCSX2.  Through
+the accepted Ch418 scanout mapping it measures MAE 20.9469 / RMSE 29.7626.
+Unlike the lower scalar score previously claimed for Ch416, this frame also
+passes the essential visual gate: its scene topology is coherent rather than
+dominated by false long triangles.
+
+## Board acceptance
+
+No new fit was required.  The timing-clean Ch423 RBF ran the new assets and
+passed every one of the 125 texture CRC, runtime-palette, staging-record,
+drain, and zero-drop gates.  The board dumped 307,200 words with 114,504
+nonzero pixels, bounds `(0,10)..(511,479)`, sum32 `0x42ee1faa`, and xor32
+`0x0dc19c66`; HDMI was switched to the resulting line-buffer scanout.
+
+The board and production RTL framebuffer dumps are byte-identical.  Both have
+SHA-256 `2bd7a81d3650c00b1397d5196713634bbd4c87a5e607b75bcf0df346cb7b3126`.
+Ch424 is accepted as the new visible baseline.