The Scala field handler matched only `val_definition`, so a mutable field
(`var b: Repo`), which parses as `var_definition`, had its type reference
silently dropped from the graph. val and var nodes are structurally identical
(both expose a `type` field), so the existing type-collection logic works
unchanged. Widen the guard to accept var_definition.
Adds a var field to the Scala fixture and a regression test.