Debug the `#[copper_runtime]` proc macro in copper-rs (`cu29_derive`): read its compile errors, decide when to run `cargo expand` / `just expand-runtime`, understand what the macro emits around the user's `App` struct, and recognise the recurring foot-guns (missing `Reflect`/`Freezable`, RON-parse-as-macro-error, `sim_mode` divergence, feature-gated lifecycle hooks). Use whenever the compiler error mentions `#[copper_runtime]`, points at the `App` struct line, or names a symbol that only exists in expanded code. For the execution model see `copper-arch`; for changing the macro implementation see `copper-core-dev`; for `just` commands see `copper-workflow`.
