feat(cypher): support label alternation MATCH (n:A|B) (#242)

openCypher node-label alternation was unsupported (parse error after the
first label). parse_node now consumes `:A|B|C` into a single "A|B|C" label
string (mirroring the existing relationship-type alternation). The seed path
unions per-label results (scan_alternation_labels), and target/hop label
filters use label_alt_matches, which ORs over the alternatives. Single-label
matching is unchanged.

Unit test (Function|Module → 5 rows; Function|Class → 4) plus a query_graph
alternation check in the smoke suite. Relationship-type alternation tests
remain green.