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

src/core/CMakeLists.txt

@@ -0,0 +1,17 @@
+# The pure substrate — see root CLAUDE.md's architecture section for the core/ purity invariant.
+#
+# Declaration order below runs base-first so the file reads as a dependency ladder; CMake
+# itself does not require it (link names resolve at generate time), and a few edges do run
+# backwards — core/wire's instrument_drop reuses the instrument's own state codec.
+
+add_subdirectory(json)
+add_subdirectory(util)
+add_subdirectory(wire)
+add_subdirectory(audio)
+add_subdirectory(model)
+add_subdirectory(capture)
+add_subdirectory(reclaim)
+add_subdirectory(version)
+add_subdirectory(view)
+add_subdirectory(ui)
+add_subdirectory(instrument)