docs: add scene switching engine documentation and update evaluation

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

evaluation.md

@@ -20,6 +20,7 @@
   - `CMD_BIND_CHANNEL`, `CMD_UNBIND`, `CMD_CYCLE`, `CMD_ADD_CHANNEL`, `CMD_REMOVE_CHANNEL` are all wired.  
   - The integration test suite now includes `test_fifo_stop_bind_unbind()` and `test_midi_channel_add()`.  
   - The FIFO pipe reader handles `"stop"`, `"bind <ch>"`, `"unbind"`, and `"add_midi"`.  
+  - **Note:** The separate test files in `tests/` (`test_audio.c`, `test_channel.c`, `test_fifo.c`, `test_loop.c`, `main.c`) are not compiled by the makefile and require a missing `test_common.h`. They are not part of the build – they do not affect functionality and may be removed in a future cleanup.  
 
 ### 2. Potential Segfaults
 - **Audio channels:** `audio_in`/`audio_out` are checked for NULL before use.