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.

CMakeLists.txt

@@ -9,43 +9,20 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+option(SERUMALT_BUILD_PLUGIN "Build production plugin and standalone formats" ON)
+option(SERUMALT_BUILD_TESTS "Build the headless QA harness" OFF)
+
+if(NOT SERUMALT_BUILD_PLUGIN AND NOT SERUMALT_BUILD_TESTS)
+    message(FATAL_ERROR "Enable SERUMALT_BUILD_PLUGIN or SERUMALT_BUILD_TESTS")
+endif()
+
 # ---------------------------------------------------------------------------
 # JUCE
 # ---------------------------------------------------------------------------
 if(NOT DEFINED JUCE_ROOT)
     set(JUCE_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/third_party/JUCE")
 endif()
-add_subdirectory("${JUCE_ROOT}")
-
-# ---------------------------------------------------------------------------
-# 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 platform-independent 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()
-
-juce_add_plugin(SerumAlt
-    VERSION 1.0.0
-    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 VST3 AU Standalone
-    PRODUCT_NAME "SerumAlt"
-    VST3_AUTO_MANIFEST ${SERUMALT_VST3_AUTO_MANIFEST})
-
-juce_generate_juce_header(SerumAlt)
+add_subdirectory("${JUCE_ROOT}" "${CMAKE_CURRENT_BINARY_DIR}/third_party/JUCE")
 
 set(SERUMALT_SOURCES
     Source/PluginProcessor.cpp
@@ -86,46 +63,33 @@ set(SERUMALT_SOURCES
     Source/GUI/Panel.cpp
     Source/Presets/FactoryPresets.cpp)
 
-target_sources(SerumAlt PRIVATE ${SERUMALT_SOURCES})
-
-target_compile_definitions(SerumAlt
-    PUBLIC
-        JUCE_WEB_BROWSER=0
-        JUCE_USE_CURL=0
-        JUCE_VST3_CAN_REPLACE_VST2=0)
-
-target_link_libraries(SerumAlt
-    PRIVATE
-        juce::juce_audio_utils
-        juce::juce_dsp
-    PUBLIC
-        juce::juce_recommended_config_flags
-        juce::juce_recommended_warning_flags)
-
-# ---------------------------------------------------------------------------
-# Headless QA harness (console app that drives the processor and checks output)
-# ---------------------------------------------------------------------------
-option(SERUMALT_BUILD_TESTS "Build the headless QA harness" ON)
-
-if(SERUMALT_BUILD_TESTS)
-    juce_add_console_app(SerumAltTest)
-    juce_generate_juce_header(SerumAltTest)
-    target_sources(SerumAltTest PRIVATE ${SERUMALT_SOURCES} Source/Tests/TestMain.cpp)
-    target_compile_definitions(SerumAltTest
+function(serumalt_configure_target target)
+    juce_generate_juce_header(${target})
+    target_sources(${target} PRIVATE ${SERUMALT_SOURCES})
+    target_compile_definitions(${target}
         PUBLIC
             JUCE_WEB_BROWSER=0
-            JUCE_USE_CURL=0)
-    target_link_libraries(SerumAltTest
+            JUCE_USE_CURL=0
+            JUCE_VST3_CAN_REPLACE_VST2=0)
+    target_link_libraries(${target}
         PRIVATE
             juce::juce_audio_utils
             juce::juce_dsp
         PUBLIC
             juce::juce_recommended_config_flags
             juce::juce_recommended_warning_flags)
+    if(MSVC)
+        target_compile_options(${target} PRIVATE /W4)
+    else()
+        target_compile_options(${target} PRIVATE -Wall -Wextra)
+    endif()
+endfunction()
+
+if(SERUMALT_BUILD_PLUGIN)
+    include(cmake/Plugin.cmake)
 endif()
 
-if(MSVC)
-    target_compile_options(SerumAlt PRIVATE /W4)
-else()
-    target_compile_options(SerumAlt PRIVATE -Wall -Wextra)
+if(SERUMALT_BUILD_TESTS)
+    enable_testing()
+    include(cmake/Harness.cmake)
 endif()