Add path and rock landscape primitives

[madawei2699]

Summary

• add PathRun for continuous, gravel, and stepping-stone paths with local waypoints
• add RockCluster for deterministic boulder and dry-garden rock groups
• add a path/rock garden sample plus docs and LLM authoring guidance

Closes #73

Tests

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

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[gemini-code-assist]

Code Review

This pull request introduces two new component types, PathRun and RockCluster, to the ComponentPlan specification and core package. PathRun allows defining deterministic local paths (such as stepping stones or gravel paths) using waypoints, while RockCluster enables the generation of deterministic groups of rough boulders. The changes include updating the Zod schemas, validation logic, expansion algorithms, and block estimation, alongside adding comprehensive documentation, tests, and a new example garden composition. There are no review comments to address, and the implementation looks solid.

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