docs(gfql): fix index_adjacency quick-start — working chain form, not unsupported Cypher

The quick-start ran a Cypher string 'MATCH (a)-[e]->(b) WHERE a.id IN $seeds
RETURN a, e, b' that hits a known limitation (#1273: row lowering supports one
MATCH source alias at a time), so it raised in test-docs and would mislead any
reader who copy-pasted it. Replace with the canonical native seeded-traversal
chain — [n({id: is_in(seeds)}), e_forward(), n()] — which is what this index page
is actually about, uses the index automatically, and runs green. Also defines the
previously-undefined my_seed_ids. Full doc-examples suite now passes locally.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lmeyerov

docs/source/gfql/index_adjacency.rst

@@ -30,15 +30,16 @@ Quick start
 .. code-block:: python
 
    import graphistry
+   from graphistry import n, e_forward, is_in
 
    g = graphistry.edges(edges_df, "src", "dst").nodes(nodes_df, "id")
 
    # Build the indexes once (out+in adjacency, plus a node-id accelerator when ids are unique)
    g = g.gfql_index_all()
 
-   # Seeded query — the index is used automatically (default index_policy='use')
-   out = g.gfql("MATCH (a)-[e]->(b) WHERE a.id IN $seeds RETURN a, e, b",
-                params={"seeds": my_seed_ids})
+   # Seeded traversal — the index is used automatically (default index_policy='use')
+   my_seed_ids = ["a", "b"]   # your seed node ids
+   out = g.gfql([n({"id": is_in(my_seed_ids)}), e_forward(), n()])
 
 ``gfql_index_all()`` is the one-liner. For finer control, build a single kind: