cursor: spatial memory navigation

Persistent cursor into the knowledge graph with navigation:
- temporal: forward/back among same-type nodes by timestamp
- hierarchical: up/down the digest tree (journal→daily→weekly→monthly)
- spatial: graph neighbor display at every position

The cursor file (~/.claude/memory/cursor) holds a single node key.
Show displays: temporal arrows, hierarchy links, semantic neighbors,
and full content. Date extraction from both timestamps and key names
handles the mixed-timestamp data gracefully.

This is the start of place cells — spatial awareness of position
in your own knowledge.

poc-memory/src/lib.rs

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