Fix preview overlap and add launch assets

[focaxisdev]

Summary

• fix the README hero overlap and align it to GitHub's 1280×640 social-preview size
• add a ready-to-upload PNG plus a short terminal demo GIF, and surface the demo in the quick start
• keep HN guidance compliant with its human-written-text rule and record the focused distribution follow-up

Validation

• npm.cmd test
• node scripts/dejavu-lint-memory.mjs --memory-root memory
• npm.cmd run check:links
• git diff --check
• strict UTF-8/no BOM/no U+FFFD checks on changed text assets
• visual inspection of the 1280×640 preview and all 8 GIF frames