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.

build_linux.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$SCRIPT_DIR"
+
+BUILD_DIR="${BUILD_DIR:-$SCRIPT_DIR/build_linux}"
+BUILD_TYPE="${BUILD_TYPE:-Release}"
+export CMAKE_BUILD_PARALLEL_LEVEL="${CMAKE_BUILD_PARALLEL_LEVEL:-2}"
+
+if [[ "$(uname -s)" != Linux ]]; then
+    echo "ERROR: build_linux.sh requires a Linux host." >&2
+    exit 1
+fi
+
+for tool in cmake ninja pkg-config "${CC:-cc}" "${CXX:-c++}"; do
+    if ! command -v "$tool" >/dev/null 2>&1; then
+        echo "ERROR: $tool not found. See README.md prerequisites." >&2
+        exit 1
+    fi
+done
+
+echo "Configuring Linux production targets in $BUILD_DIR ($BUILD_TYPE)..."
+cmake -S "$SCRIPT_DIR" -B "$BUILD_DIR" -G Ninja \
+    -DCMAKE_BUILD_TYPE="$BUILD_TYPE" \
+    -DSERUMALT_BUILD_PLUGIN=ON \
+    -DSERUMALT_BUILD_TESTS=OFF
+
+echo "Building Linux VST3 and standalone..."
+cmake --build "$BUILD_DIR" --config "$BUILD_TYPE" --target SerumAlt_VST3 SerumAlt_Standalone
+
+echo "Done. Linux artifacts: $BUILD_DIR/SerumAlt_artefacts/$BUILD_TYPE"