Moves the shared run-state atomics (currentSampleRate, block sizes,
duplexMode, run flags) into slopsmith::EngineState (src/audio/engine/) so
later extracted units take EngineState& and stay unit-testable without JUCE
devices. AudioEngine binds the members back by reference under their
historical names — zero call-site churn, behavior-identical.
The old audioRunning conflated user intent with device state (deep-read
§3/§6); it is now state.deviceRunning (same semantics, isAudioRunning compat
pinned) plus a new state.userWantsAudio written only by startAudio/stopAudio.
Nothing reads the intent flag yet — phase 8 flips setAudioDevices' restart
decision onto it.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
