perf: fix O(n²) sibling representative + batch graph edge insertion

sibling.py: replace linear scan (next over all fragments) with
dict lookup for file representative. Was O(n²), now O(n).

graph.py: use nx.DiGraph.add_weighted_edges_from() instead of
per-edge add_edge() calls. Reduces 5M function calls to one bulk
operation on large repos.

Profiling on svelte (6K files): 82s → 33s.

Author: nikolay-e

src/treemapper/diffctx/edges/structural/sibling.py

@@ -30,19 +30,16 @@ def _group_files_by_dir(self, fragments: list[Fragment]) -> dict[Path, set[Path]
         return by_dir
 
     def _build_file_representative_map(self, fragments: list[Fragment]) -> dict[Path, FragmentId]:
+        frag_by_id: dict[FragmentId, Fragment] = {f.id: f for f in fragments}
         file_to_rep: dict[Path, FragmentId] = {}
         for f in fragments:
-            self._update_file_representative(f, file_to_rep, fragments)
-        return file_to_rep
-
-    def _update_file_representative(self, f: Fragment, file_to_rep: dict[Path, FragmentId], fragments: list[Fragment]) -> None:
-        if f.path not in file_to_rep:
-            file_to_rep[f.path] = f.id
-        elif f.token_count > 0:
-            existing = file_to_rep[f.path]
-            existing_frag = next((fr for fr in fragments if fr.id == existing), None)
-            if existing_frag and f.token_count > existing_frag.token_count:
+            if f.path not in file_to_rep:
                 file_to_rep[f.path] = f.id
+            elif f.token_count > 0:
+                existing = frag_by_id.get(file_to_rep[f.path])
+                if existing and f.token_count > existing.token_count:
+                    file_to_rep[f.path] = f.id
+        return file_to_rep
 
     def _add_sibling_edges_for_dir(self, files: set[Path], file_to_rep: dict[Path, FragmentId], edges: EdgeDict) -> None:
         file_list = sorted(files)

src/treemapper/diffctx/graph.py

@@ -146,9 +146,9 @@ def build_graph(fragments: list[Fragment], repo_root: Path | None = None) -> Gra
 
     all_edges = _apply_hub_suppression(all_edges, edge_categories)
 
-    for (src, dst), weight in all_edges.items():
-        graph.add_edge(src, dst, weight)
-
+    graph._g.add_nodes_from(frag.id for frag in fragments)
+    graph._g.add_weighted_edges_from((src, dst, w) for (src, dst), w in all_edges.items() if w > 0)
+    graph._invalidate_cache()
     graph.edge_categories = edge_categories
 
     return graph