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/ch435_frame_fbmsk_lightbuffer_closeout.md

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+# Ch435 — FRAME.FBMSK light-buffer alpha
+
+## Root cause
+
+The Ch434 current-frame light chain was structurally correct but the second
+draw of the CT32 conversion stage, draw 195973, has
+`FRAME.FBMSK=0xff000000`.  The scheduler previously discarded that field when
+relocating the framebuffer, and the external LPDDR color path had no byte-mask
+transport.  Draw 195973 therefore replaced alpha across the full 256x256
+target instead of preserving the alpha produced by draw 195957.  Feeding that
+incorrect all-128 alpha target through the remaining ping-pong passes made the
+integrated frame much too dark.
+
+## Static fidelity result
+
+The first correction was an asset-boundary reconstruction.  It retained alpha
+128 on the 20,861 lit texels and alpha zero on the 44,675 black texels, then
+propagated that target through draws 196087, 196108, 196129, 196167, and
+196177.  The resulting 311-epoch `zsrt139f52` replay passed production
+simulation and the DE25-Nano runtime gates with zero drops, overflows, AXI
+errors, Z mismatches, or color mismatches.
+
+The board framebuffer is byte-identical to simulation at SHA-256
+`d0047677371a0f6e4e319458926f604a8599c92baaf34d09b8e1fd452e31662b`.
+Its mapped mean RGB `(26.38, 23.32, 21.19)` is close to the PCSX2 frame mean
+`(28.47, 23.54, 21.33)`, and the booth, lamp, rabbit, bench, and character are
+all readable.  This supersedes the over-dark f49 result and is the live HDMI
+asset baseline.  The remaining dominant visual defect is spatial speckle and
+edge noise, not global brightness.
+
+## Native RTL repair
+
+The production path now carries a four-bit byte enable derived from
+`FRAME.FBMSK` with each fragment:
+
+- the fixture generator preserves the captured upper 32 FRAME bits and
+  rejects masks that are not byte-granular;
+- the raster wrapper converts each `00` mask byte to an enabled write byte and
+  each `ff` byte to a preserved destination byte;
+- the request FIFO and color-align RAM carry the byte enable across the GS to
+  EMIF path;
+- partial opaque writes use the same ordered destination-read/modify/write
+  path as alpha blends;
+- the color stage merges disabled bytes from the destination after either the
+  GS ALPHA equation or opaque source selection.
+
+The directed color-blend regression preserves destination alpha with
+`BE=0111`, producing `0x8024180c`.  The 1,500-fragment Z/color regression also
+passes with zero errors.
+
+The regenerated authentic two-draw fixture `zsrt139l3f` contains
+`ff000000000a0000` in draw 195973's FRAME header.  Its registered-gradient
+simulation passes 22 epochs and 131,072 fragments with zero drops, overflows,
+AXI errors, Z mismatches, or color mismatches.  Most importantly, the native
+256x256 output is byte-identical in all 65,536 words to the static corrected
+target: 44,675 black/alpha-zero texels and 20,861 lit/alpha-128 texels.
+
+## Accepted fit
+
+The owner-controlled Quartus 26.1 GUI compile completed successfully on
+2026-07-19.  The accepted RBF is 4,112,384 bytes with SHA-256
+`55b23b94f51f1aab271d91aa4fa766561c044b9633146f25f4242b431f6ce902`.
+All signoff classes are nonnegative:
+
+- EMIF setup WNS `+0.083 ns`, TNS `0.000 ns`;
+- 25 MHz design setup WNS `+6.324 ns`, TNS `0.000 ns`;
+- hold minimum `0.000 ns`, with zero TNS;
+- recovery minimum `+1.074 ns`;
+- removal minimum `+0.017 ns`;
+- minimum-pulse-width minimum `+0.200 ns`.
+
+Final resources are 38,387 / 46,800 ALMs (82%), 5,327,792 / 7,331,840
+block-memory bits (73%), 344 / 358 RAM blocks (96%), and 160 / 376 DSP
+blocks (43%).
+
+## Hardware acceptance
+
+The RBF checksum was verified on the DE25-Nano and `core_loader.sh` loaded it
+successfully; `fpga0` reported `operating`.  The authentic 22-epoch
+`zsrt139l3f` fixture then passed every texture CRC, staging, fresh-drain, and
+zero-drop gate.  Its 307,200-word framebuffer has SHA-256
+`e2b9f847fb64f5dbc70f112df91c6fccc54bf439138f772927150ff99aa026ce`,
+byte-identical to production simulation.  The native target contains exactly
+20,861 lit/alpha-128 pixels and 44,675 black/alpha-zero pixels in its 256x256
+region, proving that draw 195973 preserves the destination alpha byte on
+silicon.
+
+The complete 311-epoch f52 scene was then replayed on the same RBF.  All
+epochs completed with zero fragment drops and its framebuffer remained
+byte-identical to simulation at SHA-256
+`d0047677371a0f6e4e319458926f604a8599c92baaf34d09b8e1fd452e31662b`.
+HDMI therefore ends on the readable f52 scene.  The accepted artifact is now
+the board's standard `/home/terasic/cores/retroDE_ps2.core.rbf`; the previous
+artifact is retained as `retroDE_ps2.pre_ch435.core.rbf`.