Extract world rendering to shared crate

[vanlueckn]

Summary

• Create new world-render crate with shared terrain, track, trees, and signal rendering logic
• Move 1180+ lines of rendering code from app/render.rs, app/scenery.rs, and app/signals.rs
• Update app and route-editor to depend on and use world-render
• Consolidate terrain material, shader compilation, and asset loading into one place
• Update localisation keys in both .ftl files for new terrain UI feature

Testing

• cargo test --workspace passes: 383 tests green (was 379)
• cargo clippy and cargo fmt pass cleanly
• Both app and route-editor compile and link with new crate
• Terrain display in route editor shows same ground layer as app runs
• Not run: visual diff of rendered terrain/track/signals between before/after