Phase 3a: decode captured VPX render stream to pixels (SMPTE bars)

- vpxlog.cpp: VPX_FIFODUMP=<path> records every FIFO burst ('VPXM' records)
- decode_fifodump.py: action census + payload dumps of a capture
- render_capture.py: reconstruct the DPL scene graph from a capture and
  software-render each draw_scene frame (camera, view, materials, geometry
  all taken from the wire)
- divrgb.conf + divrgb.fifodump: flyk divrgb.scn capture fixture
- divrgb-decoded.png / divrgb-frame0.png: first images ever produced from
  the Rel 4.10 VPX protocol without a real board -- the textbook SMPTE
  color-bar pattern, validating verts/conns/materials/camera in one shot
- PHASE3-PROGRESS.md: the established Rel 4.10 wire protocol (action map,
  node types, message layouts); RENDER-HARNESS.md updated

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

emulator/.gitignore

@@ -2,7 +2,12 @@
 dosbox-x/
 image/
 *.png
+# Phase 3 evidence renders (keep: decoded from the captured VPX stream)
+!divrgb-decoded.png
+!divrgb-frame0.png
 dbx_out.txt
+vpx*.txt
+sweep_*.txt
 dbx_err.txt
 src/
 vpxlog.txt