feat(compiler): parse + typecheck undirected traversal — $a <edge> $b

iss-gq-undirected-traversal, the Expand-internal design (no plan-level
Union; unblocked from iss-744/579): the grammar gains an undirected_edge
alternative in the traversal rule (angle brackets are collision-free —
comparisons live in the structurally separate filter production), the
AST Traversal carries `undirected`, Direction gains `Both`, and
typecheck resolves undirected patterns to Both after the new T22 rule:
undirected requires a same-endpoint-type edge (an asymmetric edge is
well-typed in at most one orientation, so the form is rejected with
guidance to use the directional pattern). Lowering passes Both through;
the reverse-expand orientation flip is a no-op for a symmetric
traversal. Bounds ({min,max}) and not{} compose unchanged.

Direction has no serde derives and IR never crosses a wire — no
compatibility surface. Parser/typecheck tests cover the bare, bounded,
inside-not forms, Both resolution on Knows (Person->Person), and the
T22 rejection on WorksAt (Person->Company).

crates/omnigraph-compiler/src/query/ast.rs

@@ -60,6 +60,9 @@ pub struct Traversal {
     pub dst: String,
     pub min_hops: u32,
     pub max_hops: Option<u32>,
+    /// `$a <edge> $b` — match the edge in either direction (set semantics;
+    /// same-endpoint-type edges only, enforced at typecheck).
+    pub undirected: bool,
 }
 
 #[derive(Debug, Clone)]