Motivation

• Avoid cloning the potentially large Ground data when storing it in WorldEditor by sharing ownership with Arc.
• Allow multiple parts of the generation pipeline to hold references to the same ground without copying.
• Simplify memory management and reduce allocations when passing ground into the editor and other helpers.

Description

• Change WorldEditor::ground from Option<Box<Ground>> to Option<Arc<Ground>> and add use std::sync::Arc.
• Update set_ground to accept Arc<Ground> and store it directly, and update get_ground to return Option<&Ground> using as_deref().
• In generate_world_with_options wrap the provided Ground in Arc::new(ground) and pass Arc::clone(&ground) to WorldEditor::set_ground, and update callers to use ground.as_ref() where a &Ground is required.
• Add necessary use std::sync::Arc imports and adjust call sites in src/data_processing.rs to match the new API.

Testing

• No automated tests were executed for this change.
• Repository changes were committed locally and the modified files were updated without running a build or test suite.

Codex Task