Removed the two internal closer_app_explainer_feature.{png,svg} and
closer_e2ee_privacy_feature.{png,svg} files from docs/brand/generated-art/.
They were unreferenced banner-like art referenced by no store/README
doc, and the new docs/store/feature-graphic-1024x500.png (the
white-keyhole logo heart, 122aef7b) is the single source of truth for
README/store banner work.
docs/NameChange.md §5 updated to reflect the retirement (was 'OPTIONAL,
not store-facing'; now 'Retired ... so there is a single source of
truth') and points to the new feature-graphic as the canonical banner.
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