query: peg-based query language for ad-hoc graph exploration

poc-memory query "degree > 15"
poc-memory query "key ~ 'journal.*' AND degree > 10"
poc-memory query "neighbors('identity.md') WHERE strength > 0.5"
poc-memory query "community_id = community('identity.md')" --fields degree,category

Grammar-driven: the peg definition IS the language spec. Supports
boolean logic (AND/OR/NOT), numeric and string comparison, regex
match (~), graph traversal (neighbors() with WHERE), and function
calls (community(), degree()). Output flags: --fields, --sort,
--limit, --count.

New dependency: peg 0.8 (~68KB, 2 tiny deps).

src/graph.rs

@@ -43,6 +43,13 @@ impl Graph {
         self.adj.values().map(|e| e.len()).sum::<usize>() / 2
     }
 
+    /// All edges for a node (full Edge data including rel_type)
+    pub fn edges_of(&self, key: &str) -> &[Edge] {
+        self.adj.get(key)
+            .map(|v| v.as_slice())
+            .unwrap_or(&[])
+    }
+
     /// All neighbor keys with strengths
     pub fn neighbors(&self, key: &str) -> Vec<(&String, f32)> {
         self.adj.get(key)