xint-data-dir-writes-to-release-internal-by-default

xint writes runtime data (api-costs.json, reliability-metrics.json) to its own release-internal data/ directory by default, not to any persistent path. After a version upgrade, the internal data/ gets abandoned with the old release. Fix: symlink releases//data → /opt/infra/xint/data/ immediately after install so runtime state persists across upgrades.