site: replace Terrastruct public branding

[alixander]

Summary

• replace the Terrastruct footer with D2 and Hack Club fiscal-sponsor copy plus a Donate link
• remove Terrastruct Twitter metadata and wordmark
• make icons.d2lang.com canonical in navigation and examples
• remove the accidentally committed yarn-error.log

Package identity and theme labels remain unchanged for compatibility and Wave 2.

Validation

• yarn install --frozen-lockfile
• Prettier, lint, go build ./..., and go test ./... pass
• go run main.go builds JS/CSS and serves the playground locally
• git diff --check
• new external links return HTTP 200

The aggregate make.sh reaches the unrelated README formatter and fails locally only because pandoc is unavailable; all build/test/lint jobs pass.