docs: add Memory Bank mark

[dapi]

Summary

• add the selected three-layer Memory Bank mark to the README entrance
• provide primary color, true monochrome, and compact SVG variants
• keep the current README positioning from main

Verification

• xmllint --noout for all three SVG files
• rendered the primary mark at 180 px on light and dark backgrounds
• rendered the compact mark at 32 px
• ruby tools/validate-priming-manifests.rb template/memory-bank
• memory-bank-cli lint --scope-root template/memory-bank --entrypoint template/memory-bank/README.md
• memory-bank-cli doctor --profile template
• git diff --check

Risk

Documentation and image assets only; no runtime or template contract changes.