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.

.gitignore

@@ -1,9 +1,10 @@
-# CMake build directories
+# Build output. /build/ holds the default trees; the other two patterns cover a
+# BUILD_DIR override and the trees that earlier script versions left at the root.
 /build/
 /build_*/
 /cmake-build-*/
 
-# Windows output staging
+# Windows distribution staging
 /SerumAlt_Windows/
 
 # CMake generated files
@@ -12,10 +13,15 @@ compile_commands.json
 
 # IDE and OS files
 .vscode/
-.idea/
+.idea
 *.user
 .DS_Store
 .cache/
 
-# graft's local graph cache — regenerable, not committed (run `graft build`).
+# Upstream JUCE release binaries. They ship with the vendored copy and are not
+# built here; committing them would add 22 MB of Linux executables.
+/third_party/JUCE/DemoRunner
+/third_party/JUCE/Projucer
+
+# graft's local graph cache, regenerable and not committed (run `graft build`).
 /graft/