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.

CMakeLists.txt

@@ -2,9 +2,6 @@ cmake_minimum_required(VERSION 3.22)
 
 project(SerumAlt VERSION 1.0.0 LANGUAGES CXX)
 
-# ---------------------------------------------------------------------------
-# C++ standard
-# ---------------------------------------------------------------------------
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
@@ -16,9 +13,6 @@ 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()
@@ -54,7 +48,6 @@ set(SERUMALT_SOURCES
     Source/EffectUnits/Reverb.cpp
     Source/GUI/Knob.cpp
     Source/GUI/KnobLookAndFeel.cpp
-    Source/GUI/Slider.cpp
     Source/GUI/ToggleButton.cpp
     Source/GUI/Display.cpp
     Source/GUI/WaveformDisplay.cpp