test(dedup): pin chained-collapse and kept-single-member for hyperedge rewire (#2805)

Adds the two cases the deep-dive flagged: a chained collapse (a_old, a_mid -> a)
lands directly on the final survivor (the whole fix rests on the union-find
remap being fully flattened), and a hyperedge collapsing to one distinct member
is kept, not dropped (pinning the deliberate sub-two-member policy). Adds the
CHANGELOG entry.

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

CHANGELOG.md

@@ -4,6 +4,7 @@ Full release notes with details on each version: [GitHub Releases](https://githu
 
 ## 0.9.46 (unreleased)
 
+- Fix: when node dedup merges two nodes, any hyperedge that listed the merged-away node as a member now rewires that member to the survivor instead of silently dropping it, so a grouping no longer loses participants on dedup (#2805, thanks @abhay-codes07).
 - Fix: pruning a source file now also sweeps the external-import placeholder nodes it strands at degree 0, instead of leaving them to accumulate in the node count, `GRAPH_REPORT.md`, and exports; genuinely-isolated real nodes (which carry a `source_file`) are never touched (#2807, thanks @abhay-codes07).
 - Fix: when two edges connect the same node pair with different relations, the graph builder now keeps the more specific relation (`calls`, `imports`, `inherits`, ...) instead of letting a generic `references`/`uses`/`mentions` overwrite it; previously a real `calls` could be downgraded to `references` and then dropped from the call graph (#2803, thanks @abhay-codes07).
 

tests/test_dedup_remaps_hyperedges.py

@@ -125,3 +125,22 @@ def test_an_empty_remap_changes_nothing():
     hes = [{"id": "h", "nodes": ["a", "b", "c"]}]
     _remap_hyperedge_members(hes, {})
     assert hes[0]["nodes"] == ["a", "b", "c"]
+
+
+def test_chained_collapse_lands_on_the_final_survivor():
+    """A dedup remap built from union-find is fully flattened (path-compressed),
+    so a member of a chained component (a_old -> a_mid -> a) rewires directly to
+    the final survivor in a single lookup, never to an intermediate."""
+    hes = [{"id": "h", "nodes": ["a_old", "a_mid", "b"]}]
+    # what components()/UnionFind produces: every non-winner maps to the winner
+    _remap_hyperedge_members(hes, {"a_old": "a", "a_mid": "a"})
+    assert hes[0]["nodes"] == ["a", "b"]
+
+
+def test_a_hyperedge_collapsing_to_one_member_is_kept():
+    """Sub-two-member hyperedges are kept by design (build_from_json only drops
+    the zero-valid-member case). Pin it so a future refactor doesn't silently
+    start dropping a 1-member group after a collapse."""
+    hes = [{"id": "h", "nodes": ["a_old", "a_new"]}]
+    _remap_hyperedge_members(hes, {"a_old": "a", "a_new": "a"})
+    assert hes[0]["nodes"] == ["a"]  # collapsed to one, still present