Upgrade redb 2 → 4, slim down tui-markdown

redb: add ReadableDatabase trait import for begin_read().
tui-markdown: disable highlight-code (drops syntect), fix
test deps leaking into normal dependencies.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/counters.rs

@@ -7,7 +7,7 @@
 //   search_hits: key → u64   (how often memory-search found this node)
 //   last_hit_ts: key → i64   (unix timestamp of last search hit)
 
-use redb::{Database, ReadableTable, TableDefinition};
+use redb::{Database, ReadableDatabase, ReadableTable, TableDefinition};
 use std::path::PathBuf;
 
 const SEARCH_HITS: TableDefinition<&str, u64> = TableDefinition::new("search_hits");