perf: speed up npm run build

[cqnykamp]

Summary

• speed up production TypeScript builds by excluding test-only app/api work and the e2e workspace from the root build graph
• lazy-load Doenet-heavy app routes, keep MathJax config local, and avoid pulling the shared activity helper through @doenet/assignment-viewer
• move the v0.6 -> v0.7 syntax upgrader out of the Vite bundle by copying its runtime files into public/vendor and loading them only when the upgrade action is used
• document the vendor runtime setup in the relevant files

Results

• root build:ts dropped from about 34.5s to about 31.5s on this machine
• app build:assets dropped from about 58.7s to about 37.5s on this machine after vendoring the syntax upgrader
• root npm run build dropped to about 76.6s on this machine in the best retained run