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/ch426_authentic_vertex_alpha_closeout.md

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+# Ch426 — authentic vertex-alpha repair and atmospheric composite
+
+## Root cause
+
+The scheduler generator's `--auth-color-tfx` path preserved captured RGB but
+called `bake.rgbaq_with_q()`, a legacy helper that always writes alpha
+`0xff`.  The first captured alpha fan actually uses vertex alpha `0x03` at
+the anchor and `0x00` at the remaining vertices.  Under MODULATE+TCC, forcing
+that alpha to `0xff` made the authentic additive blend roughly two orders of
+magnitude stronger and produced Ch425's white rectangles.
+
+`gs_make_sh3_scheduler_fixture.py` now packs the complete captured 32-bit
+RGBA word when authentic color is enabled.  It also fails closed if emitted
+RGBAQ alpha differs from the canonicalized source vertex.  Legacy fixtures
+without authentic color retain their old opaque-alpha behavior.
+
+## Repaired fixture
+
+`zsrt139f22` regenerates the same 53 visible PSMT8 fan draws used by Ch405,
+in three exact-state groups, with:
+
+- captured RGBAQ alpha;
+- native GS 12.4 screen coordinates;
+- parsed kick semantics, authentic scissor, and framebuffer clipping;
+- authentic Z/TEST/ZMSK and ALPHA state;
+- runtime CLUT, MODULATE color, and production bilinear sampling.
+
+The repaired geometry expands to 218 one-triangle bounded epochs.  The final
+chronological candidate `zsrt139f23` is Ch424's 125 epochs followed by the 20
+authentic CT32 darken epochs and the 218 repaired alpha epochs: 363 total.
+The rejected PSMT4 pass is not included.
+
+## Production RTL acceptance
+
+The complete replay passed:
+
+- 1,009,406 fragments fed; 990,062 passed;
+- 363 markers, zero fragment drops, zero color overflows, zero AXI errors;
+- Z mismatch 0/307,200 and color mismatch 0/245,314;
+- framebuffer SHA-256
+  `ccf574d7e7ab06a519488c187da08b8c56dce5b923b6909dafba11cad6b4ce4a`.
+
+Against the PCSX2 frame, the accepted display mapping measures MAE 20.0713 /
+RMSE 28.9639, improving on Ch424's 20.9469 / 29.7626.  The new image adds a
+soft captured atmospheric veil without the false white rectangles.
+
+A controlled no-darken replay (`zsrt139f24`, Ch424 + repaired alpha) also
+passed exactly but measured 20.0890 / 29.0439.  The chronological CT32 pass
+therefore provides a small measured improvement as well as preserving the
+captured order; `zsrt139f23` remains the accepted candidate.
+
+## Silicon acceptance
+
+No new Quartus compile was needed.  The timing-clean Ch423 RBF
+(`17d1c1b49aac4d893ed56285cf0703bf375db2ee9f472ea36862cca376ae63fe`)
+ran all 363 epochs on the DE25.  Every texture CRC, palette checksum, staged
+record count, ordered drain, and zero-drop gate passed; the host returned
+`rc=0` and enabled the HDMI line-buffer source.
+
+The board dumped all 307,200 words with 196,889 nonzero pixels, bounds
+`(0,0)..(511,479)`, sum32 `0xb59376d4`, and xor32 `0x28c61f44`.  Board and RTL
+framebuffer dumps are byte-identical and share the SHA-256 above.  Ch426 is
+accepted on silicon and is the current live HDMI baseline.