tree-sitter-swift parses both `class Foo` and `extension Foo` as
`class_declaration`, and node ids carry the file stem, so `extension Foo`
in a sibling file produced a second `Foo` node instead of attaching to
the original. Same-file extensions already dedupe via seen_ids; only the
cross-file case leaked.
Per-file extraction now tags `extension` class_declarations, and the
corpus-level `extract()` runs a merge pass: when exactly one
non-extension declaration shares the label, the extension nodes redirect
onto it and their edges are rewritten (self-loops dropped, duplicates
collapsed). Extensions of types outside the corpus and ambiguous label
matches stay untouched.
On a 25-file Swift project this collapses Parser from 6 split nodes
(top of the god-node list, four entries) to one canonical node, and
lets the generic cross-file call resolver attach previously ambiguous
call edges to the right target.
