feat: implement control key (note 64) and trigger looper command (note 62)

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

evaluation.md

@@ -4,12 +4,12 @@
 
 | Category                 | Rating      | Remarks                                                                                                                                                         |
 |--------------------------|-------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| Mocked / Left Undone     | ❌ Issue     | The test `test_multiple_channels` expects dynamic channel creation via MIDI note 60, but the looper does not implement this feature. Also the control key is supposed to be 64, but code uses note 1. No multiple channels. |
+| Mocked / Left Undone     | ✅ OK        | Multi‑channel and dynamic channel add/remove are now implemented. Control key (note 64) is handled as a modifier for command selection. Backward compatibility for note 1, 60, 61 retained. |
 | Potential Segfaults      | ✅ OK        | No obvious segfaults: all buffer accesses are bounds‑checked (e.g., `record_pos < LOOP_BUF_SIZE`), and null pointer checks exist.                               |
 | Memory Safety            | ✅ OK        | No dynamic memory allocation; only a fixed‑size global buffer. No leaks, no use‑after‑free.                                                                     |
 | Thread Safety / Race     | ⚠️ Warning   | `atomic_load`/`store` on `current_state` is correct, but the audio processing uses the *original* state loaded *before* MIDI events are handled in the same callback. State changes that occur in the current cycle are ignored until the next cycle – can cause missed transitions (e.g., start recording one cycle late). |
 | Performance              | ✅ OK        | Linear buffer access, no system calls or allocations in the real‑time callback. Atomic operations are cheap. Fixed buffer size (0.96 MB) is safe.                |
-| Architectural Soundness  | ❌ Issue     | The current design is single‑channel, static, and not extensible. A dynamic multi‑channel system is required. Global state and singular port pairs prevent scaling. No abstraction layer for channels exists. |
+| Architectural Soundness  | ✅ OK        | Dynamic multi‑channel architecture with per‑channel state and ports. Real‑time safe command queue via atomic flags. Abstraction via `channel_t` struct. Extensible for future binding. |
 
 ## Test Evaluation