Add SteppedDome component

[madawei2699]

Summary

• add SteppedDome as a bounded Minecraft-style dome primitive for landmark roofs and caps
• support filled and hollow stepped tiers, validation, budgeting, repeat expansion, material fallbacks, and source metadata
• add Taj dome reference sample plus agent-facing docs

Closes #83

Validation

• pnpm --filter @i365dev/craftdag-core test
• pnpm --filter @i365dev/craftdag-core typecheck
• pnpm lint

[gemini-code-assist]

Code Review

This pull request introduces a new SteppedDome component type, which represents a bounded Minecraft-style dome approximation made of stacked shrinking tiers. The changes include comprehensive documentation updates, a Zod schema definition, expansion and validation logic within the core engine, TypeScript type definitions, a new example component plan for a Taj-style dome study, and thorough unit tests verifying its behavior. There are no review comments, so I have no feedback to provide.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.