Add StairRun component

[madawei2699]

Summary

• add StairRun for blocky vertical circulation in multi-level builds
• validate degenerate stair runs and side-rail width with repair hints
• add a multilevel sample plus ComponentPlan, LLM, and component-design-guide docs

Closes #82

Tests

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

[gemini-code-assist]

Code Review

This pull request introduces the StairRun component type to support blocky vertical circulation in multi-level builds, updating the design guides, schemas, validation logic, and tests accordingly. The review feedback identifies a critical bug in the step placement calculations where non-integer step divisions can result in gaps at the top or bottom of the staircase, and provides an elegant fix using interval partitioning to distribute step depths evenly.

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[gemini-code-assist]

When the runLength of a StairRun is not a perfect multiple of its height, the integer division Math.floor(runLength / size.height) leaves a remainder. This results in a gap at the top (or bottom) of the staircase where no steps are generated, which can cause players to fall through or prevent the stairs from connecting to the upper landing.\n\nWe can elegantly solve this by dynamically distributing the step depths across the entire runLength using standard interval partitioning. This guarantees that the staircase perfectly spans the entire run length with zero gaps, regardless of the dimensions.

  const placements: Array<{ part: string; anchor: { x: number; y: number; z: number }; size: ComponentSize; materialRole: string; materialFallback: string }> = [];\n\n  for (let step = 0; step < size.height; step += 1) {\n    const stepStart = Math.floor((step * runLength) / size.height);\n    const stepEnd = Math.floor(((step + 1) * runLength) / size.height);\n    const currentTreadDepth = stepEnd - stepStart;\n    const start = direction === "positive" ? stepStart : runLength - stepEnd;\n    placements.push({\n      part: `step_${step}`,\n      anchor: stairPlacementAnchor(anchor, axis, start, 0, step),\n      size: stairPlacementSize(axis, currentTreadDepth, 1, crossWidth),\n      materialRole: "main",\n      materialFallback: "floor",\n    });\n  }