Reduce App → LeftPanel prop-drilling for sim bridges

[TheSevenPens]

Summary

App.svelte has ~40 bindables and nearly 1:1 onFoo → sim.setFoo wrappers; LeftPanel is mostly a pass-through.

Work

• Explore a small bridge helper, shared callbacks object, or tighter child ownership
• Keep App as state owner unless a clearer pattern emerges
• Prefer incremental refactor; do not rewrite the panel in one shot

[TheSevenPens]

Left open as follow-up. CheckboxControl/SelectControl reduced some duplication on cleanup/post-studio-hygiene; fuller App↔panel bridge still TODO.