Path-existence verbs over the call graph (#155 §1):

• .reachable_to(other_query, *, via=["call"], sanitizers=None)
• .without_passing_through(sanitizers=[...])

Call-graph path existence now; when the language backends ship dataflow/slicing, via=["dataflow"] slots into the same verbs with no API change. Every returned path carries per-path confidence derived from its edges.

Part of #155. Branch: feat/issue-<n>.