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.

cmake/moduleinfo.json.in

@@ -0,0 +1,57 @@
+{
+  "Name": "@PROJECT_NAME@",
+  "Version": "@PROJECT_VERSION@",
+  "Factory Info": {
+    "Vendor": "@SERUMALT_COMPANY_NAME@",
+    "URL": "",
+    "E-Mail": "",
+    "Flags": {
+      "Unicode": true,
+      "Classes Discardable": false,
+      "Component Non Discardable": false
+    }
+  },
+  "Classes": [
+    {
+      "CID": "@SERUMALT_CID_AUDIO_MODULE@",
+      "Category": "Audio Module Class",
+      "Name": "@PROJECT_NAME@",
+      "Vendor": "@SERUMALT_COMPANY_NAME@",
+      "Version": "@PROJECT_VERSION@",
+      "SDKVersion": "@SERUMALT_VST3_SDK_VERSION@",
+      "Sub Categories": [
+        "Instrument",
+        "Synth"
+      ],
+      "Class Flags": 2,
+      "Cardinality": 2147483647,
+      "Snapshots": []
+    },
+    {
+      "CID": "@SERUMALT_CID_CONTROLLER@",
+      "Category": "Component Controller Class",
+      "Name": "@PROJECT_NAME@",
+      "Vendor": "@SERUMALT_COMPANY_NAME@",
+      "Version": "@PROJECT_VERSION@",
+      "SDKVersion": "@SERUMALT_VST3_SDK_VERSION@",
+      "Sub Categories": [
+        "Instrument",
+        "Synth"
+      ],
+      "Class Flags": 2,
+      "Cardinality": 2147483647,
+      "Snapshots": []
+    },
+    {
+      "CID": "@SERUMALT_CID_COMPATIBILITY@",
+      "Category": "Plugin Compatibility Class",
+      "Name": "@PROJECT_NAME@",
+      "Vendor": "@SERUMALT_COMPANY_NAME@",
+      "Version": "@PROJECT_VERSION@",
+      "SDKVersion": "@SERUMALT_VST3_SDK_VERSION@",
+      "Class Flags": 0,
+      "Cardinality": 2147483647,
+      "Snapshots": []
+    }
+  ]
+}