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.

Source/GUI/AGENTS.md

@@ -4,7 +4,7 @@
 
 - Related editor implementation: `../PluginEditor.h`, `../PluginEditor.cpp`
 - Knob.h, Knob.cpp
-- Slider.h, Slider.cpp
+- KnobLookAndFeel.h, KnobLookAndFeel.cpp
 - ToggleButton.h, ToggleButton.cpp
 - Display.h, Display.cpp
 - Panel.h, Panel.cpp

Source/GUI/Slider.cpp

@@ -1,13 +0,0 @@
-#include "Slider.h"
-
-namespace serum
-{
-
-Slider::Slider (const juce::String& name, std::function<juce::String (float)> fmt)
-    : Knob (name, std::move (fmt))
-{
-    setSliderStyle (juce::Slider::LinearHorizontal);
-    setTextBoxStyle (juce::Slider::NoTextBox, false, 0, 0);
-}
-
-} // namespace serum

Source/GUI/Slider.h

@@ -1,18 +0,0 @@
-#pragma once
-
-#include "Knob.h"
-
-namespace serum
-{
-
-// ===========================================================================
-// Horizontal slider (same formatting machinery as Knob).
-// ===========================================================================
-class Slider : public Knob
-{
-public:
-    explicit Slider (const juce::String& name,
-                     std::function<juce::String (float)> formatter = {});
-};
-
-} // namespace serum

Source/PluginEditor.h

@@ -3,7 +3,6 @@
 #include <JuceHeader.h>
 #include "PluginProcessor.h"
 #include "GUI/Knob.h"
-#include "GUI/Slider.h"
 #include "GUI/ToggleButton.h"
 #include "GUI/Display.h"
 #include "GUI/Panel.h"

Source/PluginProcessor.h

@@ -22,6 +22,8 @@ public:
     void prepareToPlay (double sampleRate, int samplesPerBlock) override;
     void releaseResources() override;
     void processBlock (juce::AudioBuffer<float>&, juce::MidiBuffer&) override;
+    // Keeps the base class double-precision overload visible, so it is not hidden.
+    using juce::AudioProcessor::processBlock;
 
     juce::AudioProcessorEditor* createEditor() override;
     bool hasEditor() const override                 { return true; }

Source/Tests/AGENTS.md

@@ -16,7 +16,7 @@
 ## IF-THEN
 
 - IF a check fails THEN increment the failure count and print the preset name.
-- Build and run natively from the repository root with `./build_harness.sh --run`. Building the `SerumAltTest` target alone does not run checks.
+- Build and run natively from the repository root with `./scripts/build_harness.sh --run`. Building the `SerumAltTest` target alone does not run checks.
 - The harness-only configuration sets `SERUMALT_BUILD_PLUGIN=OFF` and `SERUMALT_BUILD_TESTS=ON`. Production scripts disable the harness.
 - The harness links the real processor and editor sources but creates no plugin bundles. Keep that build separation when adding checks.