Fixes #436.

Summary

• Keep the DevTool's initial invalid state from permanently setting the graph serializer as started.
• Allow the later @cycle/run hook to provide real app sinks after the inspected app finishes booting.
• Avoid duplicate invalid-state emissions if no compatible xstream sinks are found.

Verification

• npx --yes pnpm@7.33.7 install --frozen-lockfile --store-dir /tmp/pnpm-cyclejs-store-436
• yarn install --frozen-lockfile in devtool/
• git diff --check

Existing verification blockers

• npm run lint in devtool/ fails on existing lint violations across graphSerializer.ts, launcher.ts, panel/index.ts, and panel/model.ts (object access via string literals is disallowed, plus one shadowed name).
• npm run lib in devtool/ fails on existing TypeScript errors in old dependency declarations and existing source files, including @cycle/dom, @cycle/run, xstream, contentScript.ts, graphSerializer.ts, launcher.ts, panel/index.ts, and utils/timeSpread.ts.