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/Cargo.toml

@@ -20,6 +20,7 @@ rayon = "1"
 peg = "0.8"
 paste = "1"
 jobkit = { git = "https://evilpiepirate.org/git/jobkit.git/" }
+redb = "2"
 log = "0.4"
 ratatui = "0.29"
 crossterm = { version = "0.28", features = ["event-stream"] }