Extend the Upload final rollup step in the aggregate job to upload
src/backend/tests/benchmarks/thresholds.json alongside the existing
reports/ directory. Without this, snapshot-mode runs assemble a fresh
thresholds.json in the workspace tree but the maintainer cannot
retrieve it from the workflow artifact without a workspace-tree
workaround (A4 mitigation per 06-RESEARCH.md).
Verify-mode runs are unaffected: upload-artifact silently no-ops on
paths whose contents did not change, and thresholds.json is read-only
during verify.
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