Throws if a caller passes a path whose last two segments are both `.fusion` — that pattern only happens when a Store class joins `.fusion` onto a path that already ends in `.fusion`. Surfaces the bug at the call site instead of silently creating a stray nested directory under the project. Complements the recent in-process-runtime PluginStore fix by catching any other call paths still passing the wrong rootDir.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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