A source child can resolve outside the checkout. The output guard must
protect its canonical path before the builder checks prepared inputs.
Exclude generated dist/build trees so in-tree products can still rebuild.
Keep explicit prepared inputs protected even inside generated trees.
The public buildDesktop regression test first failed with a missing-icon
error instead of an overlap error. All 7 desktop-builder tests now pass,
including real cold/warm builds under apps/desktop/build/products.
Node syntax checks and git diff --check pass. No full suite or native
packaging ran.
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