chore: move build output under build/ and scripts into scripts/

Layout:
- scripts/ holds the four build scripts plus the new clean.sh
- cmake/ holds mingw64-toolchain.cmake
- the scripts default to build/linux, build/macos, build/windows, build/harness
- docs/ holds the audit documents, with the round 1 task list archived

Cleanup:
- scripts/clean.sh removes generated output, with --dry-run, --staging,
  --legacy and --all, and refuses tracked, protected or outside paths
- the Windows VST3 manifest now comes from cmake/moduleinfo.json.in, so its
  version, vendor and class IDs follow cmake/Plugin.cmake instead of a
  hand-written heredoc that could drift
- deleted the unused Source/GUI/Slider, which no code constructed
- fixed the hidden double processBlock overload: three warning lines to zero
- unslopped the build scripts, the two .cjs helpers and the CMake comments
- .gitignore covers the upstream JUCE Projucer and DemoRunner binaries

Verified: five scripts pass bash -n; the harness passes CTest with zero
warning lines; the Linux VST3 and Standalone link; the Windows cross-build
passes its x86-64, runtime-import and entry-point checks; each configure
exposes only its own targets; and a full build leaves Source/ and
third_party/JUCE unchanged. The generated manifest is identical to the
previous hand-written one.

scripts/build_harness.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# Build the headless QA harness. Pass --run to execute it through CTest.
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$REPO_ROOT"
+
+BUILD_DIR="${BUILD_DIR:-$REPO_ROOT/build/harness}"
+BUILD_TYPE="${BUILD_TYPE:-Release}"
+export CMAKE_BUILD_PARALLEL_LEVEL="${CMAKE_BUILD_PARALLEL_LEVEL:-2}"
+
+if [[ $# -gt 1 || ( $# -eq 1 && "$1" != --run ) ]]; then
+    echo "Usage: $0 [--run]" >&2
+    exit 1
+fi
+
+TOOLS=(cmake ninja "${CC:-cc}" "${CXX:-c++}")
+case "$(uname -s)" in
+    Linux) TOOLS+=(pkg-config) ;;
+    Darwin) TOOLS+=(xcrun) ;;
+    *) echo "ERROR: scripts/build_harness.sh requires a Linux or macOS host." >&2; exit 1 ;;
+esac
+if [[ $# -eq 1 ]]; then
+    TOOLS+=(ctest)
+fi
+for tool in "${TOOLS[@]}"; do
+    if ! command -v "$tool" >/dev/null 2>&1; then
+        echo "ERROR: $tool not found. See README.md prerequisites." >&2
+        exit 1
+    fi
+done
+
+if [[ "$(uname -s)" == Darwin ]]; then
+    xcrun --sdk macosx --show-sdk-path
+fi
+
+echo "Configuring the native QA harness in $BUILD_DIR ($BUILD_TYPE)..."
+cmake -S "$REPO_ROOT" -B "$BUILD_DIR" -G Ninja \
+    -DCMAKE_BUILD_TYPE="$BUILD_TYPE" \
+    -DSERUMALT_BUILD_PLUGIN=OFF \
+    -DSERUMALT_BUILD_TESTS=ON
+
+echo "Building SerumAltTest..."
+cmake --build "$BUILD_DIR" --config "$BUILD_TYPE" --target SerumAltTest
+
+if [[ $# -eq 1 ]]; then
+    echo "Running the QA harness with CTest..."
+    ctest --test-dir "$BUILD_DIR" --build-config "$BUILD_TYPE" --output-on-failure --no-tests=error
+fi
+
+echo "Done. QA harness: $BUILD_DIR/SerumAltTest_artefacts/$BUILD_TYPE/SerumAltTest"