fix: prevent primitive creation orphans

[Daliys]

Closes #52

What changed

• Validate create_primitive parent, transform, and material inputs before creating a GameObject.
• Accept Vector3 arrays like [9,9,9] in addition to {x,y,z} objects.
• Keep a small rollback guard around primitive creation.
• Update raw/MCP schemas, docs, and regression tests.

Validation

• bash scripts/prepush-validate.sh --static-only
• Live JSON-RPC repro: [9,9,9] succeeds; malformed [9,9] returns an error and leaves 0 orphans.
• Push hook validation passed, including quick live server smoke.