First use case: search hit tracking for rename protection. Nodes
that memory-search actively finds shouldn't be renamed.
The counters module provides increment/read/decay operations backed
by redb (pure Rust, ACID, no C deps). Next step: wire into the
poc-memory daemon via RPC so the daemon owns the DB exclusively
and memory-search sends hits via RPC.
Also reverts the JSONL search-hits approach in favor of this.
