build: split the 1423-line root CMakeLists into per-directory files with shared target helpers

CLAUDE.md

@@ -42,7 +42,7 @@ Vendors three submodules (see `.gitmodules`):
     cmake --build build
     ctest --test-dir build
 
-Every pure module has a corresponding `<module>_tests` executable target that runs without REAPER or a DAW. `CMakeLists.txt` is the authoritative target list. The two loadable-module targets are `reaper_reasampler` (the REAPER extension `.dll`/`.dylib`/`.so`) and `reasampler_vst` (the VST3 instrument; Windows-only, omitted if the `vendor/vst3sdk` slice is absent). The `sample_usage_tests` executable target runs the pure unit tests for `sample_usage` (no REAPER, no DAW).
+Every pure module has a corresponding `<module>_tests` executable target that runs without REAPER or a DAW. Targets are declared per directory: each `src/**/CMakeLists.txt` owns its own libraries and their test targets, added via `add_subdirectory` from the root, which keeps only repo-global settings (version, channel, vendor paths). `cmake/reasampler_targets.cmake` holds the two shared declaration helpers. The two loadable-module targets are `reaper_reasampler` (the REAPER extension `.dll`/`.dylib`/`.so`) and `reasampler_vst` (the VST3 instrument; Windows-only, omitted if the `vendor/vst3sdk` slice is absent). The `sample_usage_tests` executable target runs the pure unit tests for `sample_usage` (no REAPER, no DAW).
 
 ### Beta channel build
 
@@ -61,7 +61,7 @@ The VST3 target forks identically: `REASAMPLER_CHANNEL=beta` produces `reasample
 
     php vendor/WDL/WDL/swell/swell_resgen.php src/resource.rc   # macOS; Linux reuses the output
 
-Add the generated file to the appropriate `APPLE` / Linux `target_sources` block in CMakeLists.txt. The SWS extension build is the canonical reference for this step.
+Add the generated file to the appropriate `APPLE` / Linux `target_sources` block in `src/app/CMakeLists.txt`. The SWS extension build is the canonical reference for this step.
 
 ### Install / reload