Add native Go port of ELK.js 0.8.2

[alixander]

Summary

• port the 12 layout providers exposed by ELK.js 0.8.2 / ELK 0.8.1 to native Go
• expose typed graph and JSON APIs while preserving observable ordering, topology, options, Java/GWT semantics, and numeric behavior
• cover Layered compounds, ports, hierarchy, labels, self-loops, components, routing modes, and every strategy exposed by the frozen bundle
• document exact source provenance and licensing, including two Apache-licensed GWT adaptations
• add pure-Go, race, vet, Go 1.23/1.25, and pinned ELK.js differential CI

Compatibility target

This targets the exact ELK.js 0.8.2 behavior embedded by D2, not current ELK. Structural JSON output is compared exactly; only finite layout coordinates use a narrow tolerance.

The differential suite covers all registered algorithms, the valid option-family surface, 100 D2-shaped randomized graphs, 100 generic randomized graphs, and the extracted D2 ELK corpus.

Validation

• GOWORK=off go test ./...
• GOWORK=off go test -race ./...
• GOWORK=off go vet ./...
• GOWORK=off GOTOOLCHAIN=go1.23.12 go test ./...
• pinned ELK.js 0.8.2 oracle matrix
• go test -tags audit ./internal/layeredlayout
• D2 ELK corpus: 322/322 exact within the coordinate tolerance

Licensing

The port is EPL-2.0, matching ELK and ELK.js. The two translated GWT runtime routines retain their Apache-2.0 notices and license. Exact upstream commits and file mappings are recorded in PROVENANCE.md.

Companion integration: d2lang/d2#2820.

Draft only: do not merge until the companion integration is reviewed and a release version is chosen.