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/ch427_complete_alpha_family_closeout.md

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+# Ch427 — complete authentic alpha family
+
+## Objective
+
+Replace Ch426's hand-selected 53-draw alpha subset with every captured PSMT8
+alpha-fan draw in the chronological post-process range, while retaining strict
+RTL and silicon equality and admitting the result only on measured PCSX2
+fidelity improvement.
+
+## Precision control
+
+Before expanding the family, a controlled replay increased the shared
+reciprocal table from 11 to 13 bits.  It changed 32,173 output pixels, but the
+mapped PCSX2 score was effectively flat: MAE 20.0716 / RMSE 28.9629 versus
+20.0713 / 28.9639 at 11 bits.  The extra table cost therefore had no useful
+fidelity return and was rejected.
+
+## Complete family and epoch packing
+
+The captured interval contains 93 authentic PSMT8 alpha-fan draws, not the 53
+draws selected by the older Ch405 fixture.  The omitted 40 include captured
+anchor alpha as high as `0x11`; they are real atmospheric contributions, not
+transparent no-ops.
+
+An initial one-triangle expansion produced 408 alpha epochs.  It also exposed
+a valid marker corner: a fully Z-rejected epoch emits no color write, leaving
+the host's stale `frame_drained` observation high and preventing a new drain
+edge.  The acceptance gate was not weakened.  Instead, the same ordered draws
+were safely capacity-packed into 129 productive alpha epochs, with maximum
+coverage 14,497 fragments.  `zsrt139f28` is Ch424's 125 scene epochs, the 20
+captured CT32 darken epochs, and those 129 alpha epochs: 274 total.
+
+## Production RTL acceptance
+
+The complete replay passed:
+
+- 1,325,665 fragments fed; 1,306,030 passed;
+- 274 markers and drains;
+- zero fragment drops, color overflows, and AXI errors;
+- Z mismatch 0/307,200 and color mismatch 0/245,423;
+- framebuffer SHA-256
+  `2aaaabfbe4054b6445458eb43dffb5bcb9d7989c3121875c64fcf0839478d976`.
+
+Against the PCSX2 reference using the accepted display mapping, the result is
+MAE 18.9384 / RMSE 27.6464.  That is a material improvement over Ch426's
+20.0713 / 28.9639.  The complete family changes 142,182 pixels relative to the
+53-draw subset and restores broad captured atmosphere without the former
+white-frame failure.
+
+## Silicon acceptance
+
+No new Quartus compile was required.  The timing-clean Ch423 RBF
+(`17d1c1b49aac4d893ed56285cf0703bf375db2ee9f472ea36862cca376ae63fe`)
+ran all 274 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 212,213 nonzero pixels, bounds
+`(0,0)..(511,479)`, sum32 `0x9c2fcd95`, and xor32 `0x6e7390f3`.  Board and RTL
+framebuffer dumps are byte-identical and share the SHA-256 above.  The accepted
+board image is preserved as
+`captures/gs/silenthill3/extracted/sh3_zsrt139f28_ch427_board.png`.
+
+Ch427 is accepted on silicon and is the current live HDMI baseline.