What is this PR for?

Remove Helium Applications, Spells, external visualization packages, the registry/bundler, and related APIs, UI, RPCs, configuration, examples, and documentation in one change.

Preserve built-in tables/charts, ordinary interpreters, ResourcePool, and %angular. Keep legacy application metadata and chart settings when saving notes; unavailable external visualizations display as Table. Replacement visualizations are outside this change.

What type of PR is it?

Improvement

What is the Jira issue?

ZEPPELIN-6757

How should this be tested?

• Passed: JDK 11 server/upstream and shaded-interpreter builds; modern and Classic frontend production builds.
• Passed: 303 Angular shell tests, SDK typecheck, WebSocket contract check, 39 focused server tests, and common message contract tests.
• Also verified: 135 interpreter tests, visualization E2E (3 tests), server startup/API smoke checks, frontend lint, and Apache RAT.
• Full server-suite validation has 7 remaining setup errors from missing Spark/Python runtimes. Building those prerequisites on this ARM Mac is blocked by the pinned gRPC generator artifact being x86_64. The Classic WAR-related directory-access failures passed after packaging the WAR and rerunning the tests.
• Documentation site build was not run because Docker is unavailable.

Questions

• License files: no update required.
• Breaking changes: Helium packages and their APIs are removed. Rebuild and restart the server and interpreter processes together because Application RPCs/events were removed.
• Documentation: Helium-specific pages and references are removed.