counters: add redb-backed persistent counters (skeleton)

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.

poc-memory/src/lib.rs

@@ -16,6 +16,7 @@ pub mod query;
 pub mod migrate;
 pub mod transcript;
 pub mod neuro;
+pub mod counters;
 
 // Agent layer (LLM-powered operations)
 pub mod agents;