build: extract CMake plugin/harness modules and add platform build scripts

Split CMakeLists.txt into cmake/Plugin.cmake (production formats and
MinGW link flags) and cmake/Harness.cmake (console test harness with
CTest registration). Add SERUMALT_BUILD_PLUGIN and SERUMALT_BUILD_TESTS
options so production and harness builds are mutually exclusive and
each omits the other's targets.

Add build_linux.sh, build_macos.sh, and build_harness.sh with host
validation, env-overridable build dirs, and explicit option flags.
Rewrite build_windows.sh to use the checked-in mingw64-toolchain.cmake
instead of regenerating it, validate all required tools, and stage
with cmake -E copy_directory.

Broaden .gitignore to cover all build_*/ directories.

cmake/Plugin.cmake

@@ -0,0 +1,52 @@
+# ---------------------------------------------------------------------------
+# Plugin target
+# ---------------------------------------------------------------------------
+# JUCE 7.0.12 builds its VST3 manifest helper (juce_vst3_helper) with the same
+# toolchain as the plugin itself. When cross-compiling, that produces a Windows
+# .exe which cannot run on the Linux build host, so we disable the automatic
+# manifest step and inject a Windows moduleinfo.json afterwards
+# (see build_windows.sh). Native builds keep the automatic step.
+set(SERUMALT_VST3_AUTO_MANIFEST TRUE)
+if(CMAKE_CROSSCOMPILING)
+    set(SERUMALT_VST3_AUTO_MANIFEST FALSE)
+endif()
+
+set(SERUMALT_FORMATS VST3)
+if(NOT WIN32)
+    list(APPEND SERUMALT_FORMATS Standalone)
+endif()
+if(APPLE)
+    list(APPEND SERUMALT_FORMATS AU)
+endif()
+
+juce_add_plugin(SerumAlt
+    VERSION ${PROJECT_VERSION}
+    COMPANY_NAME "SerumAlt Audio"
+    IS_SYNTH TRUE
+    NEEDS_MIDI_INPUT TRUE
+    NEEDS_MIDI_OUTPUT FALSE
+    IS_MIDI_EFFECT FALSE
+    EDITOR_WANTS_KEYBOARD_FOCUS FALSE
+    PLUGIN_MANUFACTURER_CODE Salt
+    PLUGIN_CODE Sera
+    BUNDLE_ID com.serumalt.SerumAlt
+    FORMATS ${SERUMALT_FORMATS}
+    PRODUCT_NAME "SerumAlt"
+    VST3_AUTO_MANIFEST ${SERUMALT_VST3_AUTO_MANIFEST})
+
+serumalt_configure_target(SerumAlt)
+
+# ---------------------------------------------------------------------------
+# MinGW cross-build: static C++/threading runtime
+# ---------------------------------------------------------------------------
+# When cross-compiling for Windows with MinGW, link libgcc / libstdc++ /
+# winpthread statically so the VST3 DLL is self-contained. Otherwise the DLL
+# imports libgcc_s_seh-1.dll, libstdc++-6.dll and libwinpthread-1.dll, none of
+# which ship with the bundle (or exist on a stock Windows host), so the host's
+# scan would fail to load the plugin. Native (Linux) builds are unaffected.
+if(MINGW)
+    target_link_options(SerumAlt_VST3 PRIVATE
+        -static-libgcc
+        -static-libstdc++
+        -static)
+endif()