chore(examples): drop the twin-output figures no page shows

[DemchaAV]

Why

assets/readme/twin-output-pdf.png, twin-output-pptx.png and twin-output-editing.png were the figures of the README's twin-output section, which #690 removed. Nothing else uses them: no page links them and no example renders them. Their last reference was CommittedAssetDriftTest.RASTER_FIGURES, the list of rasters the drift guard cannot compare. That list alone kept about 830 KB of unused images in the tree.

What changed

• Delete the three PNGs.
• Drop their entries from RASTER_FIGURES. theOnlyAssetsThisCannotCompareAreTheOnesWrittenDown holds the files beside assets/readme/examples equal to that list, so the folder and the list change together.
• The Twin Output PDF and PPTX previews in assets/readme/examples, TwinOutputExample and its layout snapshot are unchanged.

Verification

• With the engine modules installed from this base, ./mvnw -B -ntp -f examples/pom.xml clean test reports BUILD SUCCESS: 87 tests, 0 failures, 0 skipped.
• CommittedAssetDriftTest renders the whole catalogue and passes all three of its checks:
  • every committed preview matches its render;
  • the catalogue equals the published previews plus the unpublished ones;
  • the files beside the preview folder equal RASTER_FIGURES.
• git grep -E 'twin-output-(pdf|pptx|editing)\.png' finds nothing.

Lane: test. Removes three unused README assets and one test constant; no library code.