[Epic] v4→v5 migration & LMDB→RocksDB bridge correctness

Tracking epic for the v4→v5 migration and LMDB→RocksDB bridge correctness surface: reverse (v5→v4) propagation, in-place node-by-node upgrade testing, bulk full-table-copy data loss during migration, and the bridge add_node full-copy/table-exclusion feature migrations depend on.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

This is a broad tracking epic covering v4→v5 migration, reverse v5→v4 propagation, in-place node-by-node upgrade testing, bulk full-table-copy data loss, and bridge add_node full-copy/table-exclusion migrations. Start by locating those migration and bridge entry points; done requires correctness across each listed surface, including the stated upgrade and data-loss cases.

Written by the indexing model from the issue text.