feat: add guarded symbol switching

[qqrm]

Summary

• guard websocket stream with connection id and reset domain state on symbol change
• load initial history window and ignore stale updates
• add loop test for rapid symbol switching

Testing

• cargo fmt --all
• cargo check --tests --benches
• cargo clippy --tests --benches --fix --allow-dirty -- -D warnings
• cargo test (failed: No such file or directory; wasm-bindgen-test-runner missing)
• rustup target add wasm32-unknown-unknown
• cargo test --target wasm32-unknown-unknown (failed: Exec format error)
• cargo install wasm-bindgen-cli
• CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-bindgen-test-runner cargo test --target wasm32-unknown-unknown (failed: app::tests::asset_buttons_update_current_symbol; app::tests::legend_checkbox_updates_on_renderer_change; app::tests::legend_checkbox_toggles_visibility; app::tests::timeframe_buttons_update_interval)

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https://chatgpt.com/codex/tasks/task_e_68a8200a17848332931ea9f0cd1b39a8