contrapunk-audio-chain-cutover

name	contrapunk-audio-chain-cutover
description	Wire or review feature-flagged audio-chain cutovers in Contrapunk, especially replacing the legacy built-in Synth with ElixirSynthBlock behind elixir-synth. Use when touching src-tauri/src/audio_clock.rs, src/chain/*, built-in synth routing, Chain AudioBlock setup, or A-Cut work.

Contrapunk Audio Chain Cutover

Pattern

When replacing a built-in audio component, keep the chain contract stable and switch only the block instantiation behind a Cargo feature. The router should continue sending the same SynthEvent/MIDI events unless a dedicated event bridge is explicitly part of the task.

Default Approach

1. Preserve public commands and UI behavior first.
2. Add a small constructor/helper that selects the first AudioBlock:
   • #[cfg(feature = "elixir-synth")] → ElixirSynthBlock
   • #[cfg(not(feature = "elixir-synth"))] → legacy Synth
3. Keep BlockDescriptor type IDs honest:
   • legacy: builtin.synth
   • Elixir: builtin.elixir-synth
4. Do not import root src/fx into elixir-core; share only neutral primitives through contrapunk-dsp.
5. Validate both feature configurations:
   • cargo check -p contrapunk
   • cargo check -p contrapunk --features elixir-synth
   • relevant Tauri/chain tests when touched

A-Cut Gotchas

• src-tauri/src/audio_clock.rs owns the cpal chain startup; this is where the first synth block is instantiated.
• src/chain/elixir_block.rs already adapts elixir_core::Engine to the AudioBlock trait behind elixir-synth.
• src-tauri/src/commands/engine.rs still emits SynthEvent to the built-in synth channel; if Elixir is selected through the chain, ensure events still reach the chain or add a bridge explicitly.
• Keep non-F32 cpal paths safe; they currently output silence and tick transport only.
• Avoid adding locks or allocation to the cpal callback.

Success Criteria

• Both feature-on and feature-off builds compile.
• Feature-off behavior remains legacy-compatible.
• Feature-on chain reports/uses builtin.elixir-synth and renders notes through Elixir.
• No new audio-thread allocation or blocking beyond existing chain behavior.