Add inspector overlay and instrumentation

[seregonwar]

Summary

• add a shared inspector service for capturing component trees, layout boxes, state snapshots, and frame timelines
• instrument signal updates and the renderer profiler to feed inspector data
• introduce an in-app InspectorOverlay widget and documentation for enabling or disabling it per build

Testing

• cargo fmt --all -- crates/strato-core/src/inspector.rs crates/strato-core/src/lib.rs crates/strato-core/src/state.rs crates/strato-renderer/src/profiler.rs crates/strato-widgets/src/inspector.rs crates/strato-widgets/src/lib.rs (fails: existing trailing whitespace in example sources)