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/ch433_uv_optimized_stq_closeout.md

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+# Ch433 — UV-optimized STQ packing
+
+## Root cause
+
+The repaired Ch432 sampler restored the missing perspective-bilinear samples,
+but the f19 base scene still showed granular spatial misregistration against
+its independent floating-point reference.  Post-repair isolation rejected a
+13-bit reciprocal table and rejected nearest filtering: both worsened the
+software-reference and PCSX2 scores.
+
+The remaining fixture path used `--fast-fit-scale`.  That shortcut selected
+the largest representable common S/T/Q scale, normally 6144, even though the
+scheduler generator already had a per-triangle search that scores packed RTL
+UVs against authentic floating-point UVs.  A common scale cancels only at
+infinite precision; vertex quantization, integer gradients, and reciprocal
+quantization make the choice observable.
+
+## One-variable repair
+
+`zsrt139f43` retains f19's exact 766-draw population, 254 chronological state
+groups, 125 capacity epochs, native 12.4 geometry, textures, palettes, fog,
+bilinear reference, Z state, and alpha state.  Only `--fast-fit-scale` was
+removed.  The optimizer selected scales from 0.375 through 6144 rather than
+forcing almost every triangle to 6144.
+
+The coverage planner no longer emits the shortcut by default.  It remains
+available through an explicit planner `--fast-fit-scale` option for quick
+diagnostic fixture generation.
+
+## Production result
+
+The full registered-gradient f43 replay passed all 125 epochs:
+
+- 282,363 fragments fed and 232,326 passed Z;
+- zero drops, color overflows, and AXI errors;
+- 125 complete markers and drains;
+- final Z mismatch 0/307,200 and color mismatch 0/176,888.
+
+Against its independent software reference, f43 improves from f19's
+MAE/RMSE 8.7494/18.7008 to 4.1633/11.9970.  Its mapped PCSX2 score improves
+from 18.3393/25.8374 to 17.7172/24.8787.
+
+`zsrt139f44` substitutes the optimized f43 base slices into the complete f42
+composition while preserving the chronology-correct PSMT4 character,
+direct-color highlights, and alpha/postprocess population.  The production
+replay passed all 292 epochs with 2,339,405 fragments fed, 2,285,783 passed,
+zero drops/errors, and exact final Z and color scoreboards.  Relative to f42,
+87,282 pixels change.  The mapped PCSX2 score improves from
+15.0765/21.9717 to 14.8127/21.5567.
+
+## Silicon acceptance
+
+No compile was required.  The timing-clean Ch432 RBF ran both asset-only
+fixtures.  f43 passed all 125 runtime gates and matched simulation byte for
+byte at SHA-256
+`f4ae82153791a66ae8ed7c4b3dd01f609b9706e826e971311c602181f8a08bc7`.
+
+The complete f44 board run passed all 292 texture, palette, staging, drain,
+and zero-drop gates.  It reported 240,492 nonzero words, bounds
+`(0,0)..(511,479)`, sum32 `0x96f07662`, xor32 `0x0a031310`, and selected the
+HDMI line-buffer source.  Its 307,200-word framebuffer is byte-identical to
+production simulation at SHA-256
+`3f492ab65c12415f4d0cd1aeafc64072870fd43bed237be4a923efbce30d2081`.
+
+The accepted board image is
+`captures/gs/silenthill3/extracted/sh3_zsrt139f44_ch433_board.png`; the mapped
+comparison is
+`captures/gs/silenthill3/extracted/recon/sh3_zsrt139f44_ch433_board_mapped.png`.
+Ch433 is accepted, and f44 is the current live HDMI baseline.