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/ch432_perspective_bilinear_handshake_closeout.md

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+# Ch432 — perspective palette-bilinear handshake repair
+
+## Result
+
+Ch432 removes the alternate-sample loss in the serialized perspective
+palette-bilinear raster path.  It restores a broad missing surface population
+in the unchanged 292-epoch f42 composition and is accepted on silicon.
+
+The mapped PCSX2 score improves from Ch431's MAE 18.9405 / RMSE 27.6301 to
+MAE 15.0765 / RMSE 21.9717.  This is a fidelity improvement from corrected
+RTL coverage, not an increase in draw count or a fixture substitution.
+
+## Root and repair
+
+Dense perspective epochs emitted almost exactly half of the independently
+rasterized coverage.  Isolated f19 epoch 74 produced 7,301 fragments against
+14,371 reference pixels.  The sampler advanced once when launching the held
+S1 pixel and again on `PB_RELEASE`, skipping the next S1 sample.
+
+Launch is now the single normal advance, `PB_WAIT` holds while the four-tap
+sample returns, and `PB_RELEASE` contributes a final hold cycle.  Perspective
+valid is qualified by the existing Z and texture-alpha advance enables so a
+frozen beat cannot resubmit the same pixel.  The expensive triangle-inside
+result remains off the global pipeline-enable path.
+
+An ALWAYS-Z diagnostic had previously added only 571 lit pixels and worsened
+the reference score, independently ruling out Z as the large coverage root.
+
+## Simulation acceptance
+
+The focused perspective PSMT8 regression emits all 31/31 independently
+covered samples, with no missing pixels, valid colors, correct alpha metadata,
+and correct MODULATE behavior.  The isolated production epoch-74 regression
+passes its explicit 14,341-fragment count, up from 7,301, with zero drops and
+exact scoreboards.  The affine palette-bilinear regression remains clean.
+
+The complete unchanged f42 replay passes:
+
+- 2,339,405 fragments fed and 2,285,783 passed;
+- 292 markers and drains;
+- zero fragment drops, color overflows, and AXI errors;
+- Z mismatch 0/307,200 and color mismatch 0/245,760;
+- framebuffer SHA-256
+  `31d21fb5c38816ef99ace1d7b55186dc749720e874ecb055a68b21584ec27037`.
+
+The repaired f19 scene improves against its independent software reference
+from MAE 11.7491 / RMSE 24.6649 to MAE 8.7494 / RMSE 18.7008.  Its mapped
+PCSX2 score improves from 20.9469 / 29.7626 to 18.3393 / 25.8374.
+
+## Fit and silicon acceptance
+
+The owner GUI fit is timing-clean.  Runtime-fabric audit reports 38,367 ALMs,
+336 RAM blocks, required feeder/texture/Z hierarchy present, and 310 MHz setup
+met at +0.069 ns.  Hold is nonnegative.  The deployed RBF SHA-256 is
+`69b1f91b26d0d78ba654b3d0ccc48f747fa3249f08e2a23535007ff3df40d63e`.
+
+After explicit loader confirmation, the DE25 completed all 292 epochs with
+every texture checksum, palette checksum, staged-record count, ordered drain,
+and zero-drop gate passing.  The host returned `rc=0`, selected the HDMI
+line-buffer source, and dumped 307,200 words with sum32 `0x8304eb9b`, xor32
+`0x89a68e4f`, 240,414 nonzero 32-bit words, and bounds `(0,0)..(511,479)`.
+
+The board dump is byte-identical to the production-gradient simulation at the
+framebuffer SHA above.  The accepted board image is
+`captures/gs/silenthill3/extracted/sh3_zsrt139f42_ch432_board.png`; its PNG
+SHA-256 is
+`45d7f3eed485ab4650bcf61e849598423c46302c42465dbd2a2579ae4518fab3`.
+
+Ch432 is accepted and is the current live HDMI baseline.