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/ch403_authentic_vertex_modulation_closeout.md

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+# Ch403 — authentic vertex-color modulation
+
+## Objective
+
+Make the 224139 static-frame fixture use the captured GS lighting state instead
+of drawing every texture at raw DECAL brightness.  Geometry, draw order,
+runtime CLUT handling, persistent Z, and the Ch402 40-epoch staging partition
+remain unchanged.
+
+## Finding
+
+The selected draws carry `TEX0.TFX=MODULATE` and non-unity per-vertex RGB.
+Ch402 deliberately forced `TFX=DECAL` and emitted zero placeholder RGB, which
+made the composite much brighter than the PCSX2 frame.
+
+The first Ch403 fixture proved a second, narrower gap: the existing MODULATE
+implementation was only connected to the combined affine texture path.  These
+SH3 triangles use the five-cycle perspective path, whose emit was explicitly
+hardwired to the raw texel.  Consequently the live board completed every gate
+but produced a framebuffer byte-identical to Ch402.
+
+## Implementation
+
+- `tools/gs_make_sh3_scheduler_fixture.py`
+  - adds opt-in `--auth-color-tfx`;
+  - preserves authentic `TEX0.TFX` and per-vertex RGB;
+  - interpolates RGB when clipping triangles;
+  - applies the same GS `texel * vertex / 128` saturation rule in the
+    independent reference.
+- `rtl/gif_gs/gs_stub.sv`
+  - aligns the already-computed S2 Gouraud color with the perspective texel by
+    four registers;
+  - applies MODULATE at the perspective S1+5 emit;
+  - shares the existing three-channel modulation datapath between affine and
+    perspective paths, avoiding a duplicate multiplier set;
+  - leaves DECAL output byte-identical.
+- `sim/tb/top/tb_top_psmct32_sh3_zint640_shared.sv`
+  - verifies the full 64-bit staged header and committed TEX0 state;
+  - fixes late-epoch diagnostic fill accounting.
+
+## Fixture
+
+`zsrt139a9` retains all 68 Ch402 draws in 40 staging-safe epochs.  The largest
+list is 1844 of 2048 words (204 words headroom).
+
+The independent modulated reference is:
+
+`captures/gs/silenthill3/extracted/recon/sh3_zsrt139a9_ref.png`
+
+Against the PCSX2 224139 image resized to the same 640x480 comparison grid,
+the fixture-only reference improves RGB MAE from 31.34 to 23.31 and RMSE from
+45.94 to 33.61.  This does not claim full fidelity: missing draws/fog and the
+perspective/XY precision deficit remain visible.
+
+## Verification
+
+- Generator preflight: 40 epochs, max 1844 words, no staging overflow.
+- Focused epoch 39 MODULATE proof:
+  - old DECAL hash: `37401414dd6ee3b2888d4ddb4aff98903900475e6cf2a47d973959ee05d05fb8`
+  - MODULATE hash: `c85edea3d42e983a31f630b7fffc1457c26233492c332860b7913c20ad7613c0`
+  - average nonblack RGB sum: 258.49 -> 113.23; geometry remains present.
+- Focused Ch402 DECAL regression remains byte-identical at the old hash.
+- Full 40-epoch RTL:
+  - 649,189 fragments fed; 332,984 passed persistent Z;
+  - 40 markers, 40 rises / 39 falls, zero drops/overflows/BRESP errors;
+  - Z mismatch `0/307200`; color mismatch `0/131618`;
+  - framebuffer SHA-256
+    `d286e9d5839fc7ab63ae6bc1aa3c26fa867397171ad3d1a5e5edc3b6c5b14964`;
+  - PASS.
+- The final shared-multiplier implementation reproduces the focused MODULATE
+  hash exactly and its late-epoch diagnostic passes with zero errors.
+- Owner Quartus 26.1 GUI fit:
+  - setup worst slack `+0.080 ns`, TNS `0`;
+  - hold passes, worst reported slack `0.000 ns`, TNS `0`;
+  - 37,353 / 46,800 ALMs, 53,238 registers, 284 / 358 RAM blocks,
+    4,177,064 memory bits, and 153 / 376 DSP blocks;
+  - core RBF SHA-256
+    `28659ebfccb19f03478fcd105904cc1b03195e8cc0907fad3903f56d7821975b`.
+- Live-board acceptance:
+  - all 40 epochs completed with every texture CRC, palette checksum, staged
+    word/record count, fresh-drain, fragment-drop, and bus-response gate clean;
+  - host runner returned `0`;
+  - the 307,200-word board framebuffer compares byte-for-byte with RTL;
+  - board and RTL framebuffer SHA-256
+    `d286e9d5839fc7ab63ae6bc1aa3c26fa867397171ad3d1a5e5edc3b6c5b14964`.
+
+## Status
+
+Complete and accepted on silicon.  The current HDMI image is the authentic
+vertex-color-modulated 40-epoch composite; the next fidelity work is geometry
+precision and remaining GS effects/draw coverage, not Ch403 correctness.