Update JUCE to 9.0.4, the roadmap, and the build docs

[drowaudio]

JUCE 9.0.4

• CMakeLists.txt: fetch JUCE 9.0.4 (was 9.0.3).
• build.yaml: the "as dependency" matrix tests 8.0.13 and 9.0.4 (was 9.0.3). JUCE 8 stays on 8.0.13.
• tests/pluginval_as_dependency/CMakeLists.txt: default JUCE_VERSION 9.0.4.
• CHANGELIST.md: "Updated JUCE to 9.0.4".

9.0.4 includes LV2 hosting, MessageManagerLock and Linux windowing fixes, plus protection against malformed String input.

Roadmap

• Ticked: acceptance testing, integration with CTest (tests/AddPluginvalTests.cmake, available since v1.0.4).
• Added as done: machine-readable reports (JUnit XML / JSON), GitHub Actions.
• Added as open: the remaining acceptance-testing items (parameter automation, more comparison methods, multiple tests per config file).
• "Improved stack trace/crash reporting" left open.

Build docs

JUCE is fetched by CPM at configure time; the repo has no submodules.

• README.md: removed git submodule update --init from the local build instructions. (The instructions for adding pluginval to another project as a submodule are unchanged.)
• CLAUDE.md: removed the modules/juce submodule entry and the submodule build step, dropped the PLUGINVAL_FETCH_JUCE option (it doesn't exist in CMakeLists.txt), and said where JUCE comes from.

Testing (macOS, clean Debug build)

• CPM fetched JUCE 9.0.4 (JUCE v9.0.4 in --version).
• run-tests passed, the 5 acceptance CTests passed, and the validate action passed all 4 dogfood plugins.
• No new compiler warnings: the 4 that appear are in pluginval's own sources and unrelated to JUCE.