The write-time mirror did not eliminate the leak in field testing. Re-add the
DIAG diagnostics around the exit-node selection flow to capture a fresh trace:
- UpdateRoutes: incoming client networks, selector state before/after the
management update, and the networks remaining after FilterSelectedExitNodes.
- mirrorV6ExitPairSelections: the NetIDs present in this update and the v6 pairs
V6ExitMergeSet derives from them (reveals whether the v4 base and its ::/0 pair
are present in the same update so the pair can be matched).
- SyncPairedSelection: the base/paired state before and after the sync.
- FilterSelectedExitNodes / applyExitNodeFilter: per-route SKIP/KEEP/DROP and the
selection lookups behind each decision.
- updateExitNodeSelections / logExitNodeUpdate: categorization and deselect set.
Temporary; to be removed once the root cause is confirmed.
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