docs: refresh README with hero

[rita-aga]

Summary

• replace the README with the refreshed narrative and hero treatment
• keep only the hero asset in-repo; README diagrams are ASCII instead of SVG/PNG mock assets

Validation

• Rendered README with npx --yes marked -i README.md -o /tmp/temper-clean-readme-pr-preview.html --gfm
• Pre-push hook passed rustfmt, clippy, and readability ratchet
• Full local test suite reached temper-actor-runtime integration tests and failed because local testcontainers could not start Postgres: Client(CreateContainer(RequestTimeoutError))