Add delay and reverb FX section with full DSP

Rename the DIST section to FX and expand it with tempo-synced delay
(Time/Strength/Sync) and Schroeder reverb (Roomsize/Strength/Diffusion).
New FxProcessor runs post-voice on the stereo bus; widen the editor to
1280px to fit the new section. Add a TestHost check that the FX tail
rings out after note release.

CMakeLists.txt

@@ -84,6 +84,7 @@ target_sources(Monostep PRIVATE
     Source/PluginProcessor.cpp
     Source/PluginEditor.cpp
     Source/ui/SequencerMatrix.cpp
+    Source/dsp/FxProcessor.cpp
 )
 
 target_include_directories(Monostep PRIVATE Source)
@@ -108,6 +109,7 @@ if(MONOSTEP_BUILD_TESTS)
     target_include_directories(MonostepTestHost PRIVATE Source)
     target_sources(MonostepTestHost PRIVATE
         Source/PluginProcessor.cpp
+        Source/dsp/FxProcessor.cpp
         tests/TestHost.cpp
     )
     target_compile_definitions(MonostepTestHost PRIVATE