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/ch357_audit_log.md

@@ -0,0 +1,59 @@
+# Ch357 audit log — NATIVE 640×480 LPDDR framebuffer
+
+Codex rung after Ch356. The Ch356 N-texture scheduler on a **native 640×480** LPDDR framebuffer. Draws land at their
+**authentic screen coordinates** (the Ch356 union-origin translation is removed). The scheduler and host are **unchanged**
+from Ch356 — only the framebuffer geometry differs. Same 3 epochs (dump order): E0=idx11671, E1=idx19562, E2=idx89761.
+
+## Geometry (Codex's spec — all gated in the fixture generator, fail-closed)
+- FBW=10, stride **2560 B**, FB size **0x12C000** (1,228,800 B), **80 beats/row**, **38,400 beats/frame**.
+- authentic coords (no union translation): draws span x[170..490] y[9..390] — fit inside 640×480 (gated: fail-closed if any vertex escapes).
+- FB region **[0, 0x12C000) disjoint** from the texture region **[0x200000, 0x240000)** (gated).
+- texture kept at 0x200000 (single-region cache), unchanged from Ch356.
+
+## Tooling — `gs_make_sh3_scheduler_fixture.py --fb640 --emit`
+- new `--fb640` mode: native 640×480, OX=OY=0 (no translation), geometry/fit/disjoint gates, distinct fixture prefix
+  `sh3_s640` so Ch356 (`sh3_sched`) and Ch357 (`sh3_s640`) fixtures coexist. Ch356 output byte-identical (verified).
+- emits: feeder_sh3_s640{k}.mem, sh3_s640{k}_tex_lpddr/idx/pal/refmap.mem, composed sh3_s640_refmap.mem, bootlet
+  bios/payload_sh3_s640 (preloads all N relocated CLUTs), sh3_s640_params.vh, descriptor table sh3_s640_epochs.txt.
+- composed-reference pixel counts (42414 covered / 6124 multi-epoch) MATCH Ch356 exactly — translation-invariant, a good cross-check.
+- Q-canonicalization self-test PASS.
+
+## Integration sim (tb_top_psmct32_sh3_sched640) — PASS
+- preclear once; 3 FRESH cache fills (CRC 0x26a88b77/0x95a83cbc/0xfbdeaa32, beats=8192, rd_errs=0); 3 lists STREAMED via
+  the feeder write port (word0=68 each); 3 FRESH ORDERED drains (E0 low→high; E1/E2 high→low→high); records=68 each; ovf=0.
+- scanout held off until the last drain; **scanout beats/frame = 38400 EXACT** (the PASS verdict gates on this `$error` check);
+  underflow=0, rd_errs=0; full 640×480 frame scored.
+- oracle: ALL=94.1% ≤1texel, MULTI(≥2)=92.7%, **clut_bad=0** (reciprocal-limited fidelity, same as Ch356; not accumulation).
+
+## Accumulation correctness — 100% BIT-FOR-BIT on the FULL 640×480 frame (`make tb_top_psmct32_sh3_sched640_compose`)
+- render ALL + each epoch in isolation (+ONLY=k +FBDUMP), composite (nonzero-wins DECAL, compose_sched.py --width 640),
+  compare to the joint ALL render.
+- **307200/307200 px, 0 mismatch — 100.0000% BIT-FOR-BIT.** Full-frame reference; accumulation exact on the native FB.
+
+## Host — UNCHANGED (proves "scheduler unchanged")
+- the SAME `ps2_sh3_sched` binary drives the 640 descriptor table: reads `fbwords` from the META line (307200), preclears
+  0..0x12C000, iterates the same per-epoch flow. Dry-run green (all 3 CRCs, 619 words each, rc=0).
+- BOARD RUN: `sudo ./ps2_sh3_sched sh3_s640_epochs.txt`.
+
+## Board prep — fit-ready, owner-gated
+- de25 top: new `GS_SH3_LPDDR_FB_640` branch → SCANOUT_LB 2560/80/480 + FEEDER_STG_WORDS=2048. Elaborates clean under
+  GS_SH3_LPDDR_FB+_640+GS_LPDDR4B_FB+GS_LPDDR_TEX+USE_QSYS_TOP (only the known Quartus-only EMIF_Qsys unresolved in iverilog).
+- profile `sh3_lpddr_fb_640`: fixtures bios/payload_sh3_s640; macros GS_SH3_LPDDR_FB + GS_SH3_LPDDR_FB_640 + GS_LPDDR_TEX.
+  QSF verified (macros + fixtures applied; no stale SCHED/MULTITEX). NO FEEDER_STG_INIT_FILE (runtime-staged).
+
+## Copyright
+- `sh3_s640_epochs.txt` added to the exact-path ignore rule; all `sh3_s640*` fixtures + build FB dumps gitignored; only own
+  code (TB, generator, host, compose) trackable.
+
+## Regression — strict tally (full `make run`)
+- `make` exit **0**; completion marker `=== all testbenches complete ===` present.
+- **272 / 272** run-list TBs → PASS verdict (the 7 "missing" are name-aliases; their `_stub` targets confirmed PASS).
+- **0** `] FAIL` verdicts (the 2 `FAIL-pix` hits are per-pixel diagnostic fields; both TBs verdict PASS, errors=0).
+- **0** `$fatal` / assertion failures. The lone `errors=128` is the pre-existing `tb_gs_lpddr_scanout_lb` internal diagnostic
+  counter (unmodified by Ch357; TB verdicts PASS). The many "error" grep hits are benign iverilog lint (`$error` un-synthesizable).
+- Identical tally to the Ch356 regression → Ch357 introduced **zero regressions**.
+
+## Claim
+- Ch356 scheduler on a **native 640×480 framebuffer with authentic screen coordinates**. Accumulation 100% bit-for-bit on the
+  full frame; exact 38400 scanout beats; FB/texture ranges disjoint. Fidelity ~92–94% (reciprocal floor, unchanged). Census
+  Z usage next; no speculative Z.