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/ch434_current_frame_lightbuffer_closeout.md

@@ -0,0 +1,80 @@
+# Ch434 — current-frame light-buffer reconstruction
+
+## Fidelity gap
+
+Ch433's accepted `zsrt139f44` frame used the captured contents of TBP 11264
+for the two late full-screen darkening sprites.  That texture was stale GS
+local memory rather than the intermediate produced by the current frame.
+The result retained recognizable scene structure, but its lighting was not
+chronologically connected to the scene being rendered.
+
+The omitted current-frame chain is:
+
+1. a PSMCT16S light target at FBP 256/TBP 8192, built by a clear, 2,053
+   untextured Z-tested additive/subtractive volume draws, and a final bias
+   sprite;
+2. conversion to the CT32 target at FBP 320/TBP 10240;
+3. three CT32 ping-pong sprites through FBP 352/TBP 11264 and FBP 320;
+4. the authentic draws 196167 and 196177, which sample the final TBP 11264
+   image and blend it across the display.
+
+Ch434 reconstructs this chain as ordered static fixtures.  The PSMCT16S
+target is rendered with the accepted scene Z, decoded to linear CT32, and
+then passed through the captured CT32 sprite states before the two display
+composites.  This is a static fidelity bridge; animated render-to-texture
+still requires native target residency and ping-pong ownership in RTL.
+
+## Triangle-record topology repair
+
+The first integrated attempt exposed a fixture-contract bug.  The scheduler
+generator expanded captured TRI_STRIP, TRI_FAN, and SPRITE topology into
+independent three-vertex records, while preserving the captured PRIM type in
+the feeder header.  Because the feeder reissues PRIM for every record, a
+SPRITE record was assembled as a two-vertex sprite plus a dangling vertex,
+and strip/fan records restarted with the wrong topology.
+
+`tools/gs_make_sh3_scheduler_fixture.py` now advertises TRIANGLE for every
+expanded three-vertex record while preserving authentic TME, ABE, FST, and
+CTXT state.  This changed the broken all-white ping-pong result into a stable
+binary light mask that retains the booth and character geometry through all
+three copies.
+
+## Rejected diagnostics
+
+- `zsrt139f46` painted the untextured volume family directly into the display.
+  It was useful for isolation but architecturally wrong because the family
+  targets PSMCT16S off-screen storage.  It was never deployed.
+- `zsrt139f48` used the reconstructed chain before the triangle-record repair.
+  It passed the internal RTL scoreboard but collapsed the intermediate to
+  white and over-darkened the display.  Its mapped PCSX2 score was
+  MAE/RMSE 19.4763/29.3660, so it was rejected and never deployed.
+
+## Accepted result
+
+`zsrt139f49` combines the accepted Ch433 scene, the topology-correct
+current-frame light composite, and the existing textured-alpha tail.  The
+production registered-gradient simulation passed all 311 epochs:
+
+- 2,339,914 fragments fed and 2,286,292 passed;
+- zero fragment drops, color overflows, and AXI errors;
+- 311 complete markers and drains;
+- zero final Z and color scoreboard mismatches.
+
+Against the PCSX2 frame through the accepted display map, f49 scores
+MAE/RMSE 14.1067/21.3468.  This improves both metrics from f44's
+14.8127/21.5567, with a 0.7060 MAE reduction.
+
+No compile was required.  The timing-clean Ch432 RBF ran the new assets on
+the Terasic board.  All 311 runtime texture, palette, staging, drain, and
+zero-drop gates passed.  The host reported 244,981 nonzero words, bounds
+`(0,0)..(511,479)`, sum32 `0x385dc4f6`, xor32 `0x6330653c`, and selected HDMI
+line-buffer source 1.
+
+The 307,200-word board framebuffer is byte-identical to production
+simulation at SHA-256
+`adb21828465758b0601956abef39b649f1f441a0a51831f90826ae76936f6606`.
+The accepted board image is
+`captures/gs/silenthill3/extracted/sh3_zsrt139f49_ch434_board.png`; its mapped
+comparison is
+`captures/gs/silenthill3/extracted/recon/sh3_zsrt139f49_ch434_board_mapped.png`.
+Ch434 is accepted, and f49 is the current live HDMI baseline.