Document architecture roadmap and tidy dependencies

[qqrm]

Summary

• add a staged architecture refactoring roadmap so we can break the app into focused crates without cycles
• drop the unused backtrace dependency and teach cargo-machete to ignore the macro-only wasm-bindgen-futures

Testing

• cargo fmt --all
• cargo check --tests --benches
• cargo clippy --tests --benches --fix --allow-dirty -- -D warnings
• CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-bindgen-test-runner cargo test (fails: wasm-bindgen tests expect a browser DOM; Node runner lacks document.body())
• cargo machete

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https://chatgpt.com/codex/tasks/task_e_68cc1540ae3c8332a533f7d87523ea40