dsp/: C++ skeleton with working FFT/WOLA STFT

- Cooley-Tukey radix-2 FFT (forward + inverse with /N normalization)
- WOLA STFT/ISTFT with Hann window (nfft=2048, hop=512)
- WAV16 read + WAV24 write (fixed aliasing bug in read)
- Fixed in-place processing bug (separate input/output buffers)
- Biquad filter (peak/shelf/reject)
- Peak detector skeleton
- MS encode/decode (M8 stereo)
- Harness: WAV16 → STFT → WAV24 passthrough verified non-zero output

Verified: burst500.wav passthrough produces output RMS=0.0678

dsp/CMakeLists.txt

@@ -0,0 +1,32 @@
+cmake_minimum_required(VERSION 3.10)
+project(soothe2_dsp)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+find_package(Threads REQUIRED)
+
+add_library(soothe2_dsp SHARED
+    dsp/fft_plan.cpp
+    dsp/fft_stage.cpp
+    dsp/cody_waite.cpp
+    dsp/twiddle_builder.cpp
+    dsp/fft.cpp
+    dsp/spectral.cpp
+    dsp/filter.cpp
+    dsp/detect.cpp
+    dsp/ms.cpp
+    dsp/harness.cpp
+)
+
+target_include_directories(soothe2_dsp PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_link_libraries(soothe2_dsp Threads::Threads)
+
+set_target_properties(soothe2_dsp PROPERTIES
+    POSITION_INDEPENDENT_CODE ON
+)
+
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    target_compile_options(soothe2_dsp PRIVATE -O3 -march=native)
+endif()