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/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.