fix d2js CJS asset paths

[alixander]

Summary

• define the Node bundle asset directory at bundle runtime instead of allowing Bun to inline the source file URL
• use the runtime directory for WASM, worker, and ELK assets in both ESM and CJS builds
• mark the generated CJS output directory as CommonJS so Node can load the package through its require export
• preserve browser behavior and package layout

Why

Bun 1.3.14 inlined import.meta.url in the CommonJS bundle as the source path. The CJS integration test then looked for wasm_exec.js outside d2js/js, blocking the stage-only npm workflow before it could test trusted publishing. A packed-package review also found that Node treated the generated CJS .js files as ESM because the root package uses type: module; the nested marker makes the declared require export work in Node.

Verification

• git diff --check
• formatted source verified by the Linux build
• Linux Bun 1.3.14 run on the banner fix: 32 unit tests passed; CJS and ESM integration both passed
• current Bun 1.3.14 local integration suite: 2 passed, 0 failed
• packed @terrastruct/d2 tarball installed in an isolated directory; both CommonJS and ESM builds compiled x -> y under Node 24
• packed tarball contains dist/node-cjs/package.json with the CommonJS marker
• local macOS unit baseline was unchanged at 20 pass / 12 fail; the pre-patch build produced the same result

Nothing is published or staged by this PR.