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/ch404_chronological_character_detail_closeout.md

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+# Ch404 — chronological character-detail tail
+
+Ch404 appends the next 36 authentic opaque draws after the accepted Ch403
+frame. Thirty-five staging-safe epochs preserve dump order and cover the
+character/lower-body region without changing RTL or requiring another fit.
+
+## Fixture and infrastructure
+
+- `zsrt139b1`: generated 35-epoch detail tail.
+- `zsrt139b2`: merged Ch403 base plus the tail, 75 epochs total.
+- `tools/merge_runtime_sched.py`: concatenates accepted runtime-CLUT fixtures
+  without regenerating or changing their staged records.
+- Host and shared-testbench epoch ceilings increase from 64 to 128; staging RAM
+  remains 2,048 words and the FPGA image is unchanged.
+
+## RTL verification
+
+- 667,279 fragments fed; 351,074 passed persistent Z.
+- 75 ordered markers and drains.
+- Zero request drops, color FIFO overflows, or AXI response errors.
+- Z mismatch `0/307200`; color mismatch `0/132783` written pixels.
+- RTL framebuffer SHA-256:
+  `8715381278c2b0a6e527329fb02aaaaf53edbaefd637f115723a3425bbd1b209`.
+
+The tail adds 1,003 nonblack pixels over Ch403. Against the resized PCSX2
+frame, RGB MAE improves from 23.8474 to 23.7846 and RMSE from 34.8791 to
+34.6956. This is a bounded character-detail gain, not the missing environment
+or fog solution.
+
+## Silicon acceptance
+
+The timing-clean Ch403 RBF was retained. The board completed all 75 epochs
+with every texture CRC, runtime palette checksum, staging count, record count,
+fresh drain, zero-drop, and bus-error gate clean; the host returned `0`.
+
+The 307,200-word board framebuffer is byte-for-byte identical to RTL and has
+the same SHA-256
+`8715381278c2b0a6e527329fb02aaaaf53edbaefd637f115723a3425bbd1b209`.
+The 75-epoch composite is live on HDMI.