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.

AGENTS.md

@@ -62,11 +62,12 @@ safety fixes practical, without adding a generic framework.
 Read README.md's Building section before changing scripts, dependencies, or CMake.
 It is the maintained source for prerequisites, options and artifact paths.
 
-- Linux production: `./build_linux.sh`.
-- macOS production, on a Mac: `./build_macos.sh`.
-- Windows x64 VST3, cross-compiled on Linux: `./build_windows.sh`.
-- Native QA build and execution: `./build_harness.sh --run`.
-- Shell syntax: `for script in build_*.sh; do bash -n "$script"; done`.
+- Linux production: `./scripts/build_linux.sh`.
+- macOS production, on a Mac: `./scripts/build_macos.sh`.
+- Windows x64 VST3, cross-compiled on Linux: `./scripts/build_windows.sh`.
+- Native QA build and execution: `./scripts/build_harness.sh --run`.
+- Shell syntax: `for script in scripts/*.sh; do bash -n "$script"; done`.
+- Build output removal: `./scripts/clean.sh --dry-run`, then the flags you need.
 
 Production scripts explicitly enable `SERUMALT_BUILD_PLUGIN` and disable
 `SERUMALT_BUILD_TESTS`. The harness script does the reverse in a separate directory.
@@ -74,10 +75,11 @@ Building `SerumAltTest` alone does not execute it; `--run` invokes CTest.
 When changing CMake, verify production has no `SerumAltTest` target and the
 harness-only build has no `SerumAlt`, VST3, AU, or Standalone target.
 
-Keep existing build trees and local changes intact. Use a new `BUILD_DIR` for
-clean-build verification. Use a fresh Windows `OUTPUT_DIR` for distribution checks,
-since staging does not remove extra files. Never edit generated CMakeCache.txt files.
-The checked-in MinGW toolchain is the single source of cross-compiler configuration.
+Keep existing build trees and local changes intact. Each script defaults to a
+directory under `build/`; use a new `BUILD_DIR` for clean-build verification. Use a
+fresh Windows `OUTPUT_DIR` for distribution checks, since staging does not remove
+extra files. Never edit generated CMakeCache.txt files. The checked-in
+`cmake/mingw64-toolchain.cmake` is the single source of cross-compiler configuration.
 
 ## Current source layout
 
@@ -95,7 +97,7 @@ The lowercase `Source/engine`, `modulation`, `params`, `plugin`, and `synth`
 directories currently hold guidance, not relocated implementations. Consult their
 `AGENTS.md` files when editing the related root-level sources. Consult the matching
 `AGENTS.md` when editing an existing implementation subdirectory.
-`AUDIT_REPORT.md` is a historical audit and proposed refactor, not a pending
+`docs/AUDIT_REPORT.md` is a historical audit and proposed refactor, not a pending
 instruction to move files. Update the explicit source list and includes only when
 an actual source move is part of the requested task.