perf(retrieve): increase GLOBAL_SEARCH_TOPK from 5 to 10

More vector candidates for reranker to evaluate = better precision.
With Cohere rerank-v3.5, 10 candidates gives the cross-encoder enough
material to find the best match without excessive latency.

none

Author: Dicoangelo

openviking/retrieve/hierarchical_retriever.py

@@ -48,7 +48,7 @@ class HierarchicalRetriever:
     MAX_RELATIONS = 5  # Maximum relations per resource
     SCORE_PROPAGATION_ALPHA = 0.5  # Score propagation coefficient
     DIRECTORY_DOMINANCE_RATIO = 1.2  # Directory score must exceed max child score
-    GLOBAL_SEARCH_TOPK = 5  # Global retrieval count
+    GLOBAL_SEARCH_TOPK = 10  # Global retrieval count (more candidates = better rerank precision)
     HOTNESS_ALPHA = 0.2  # Weight for hotness score in final ranking (0 = disabled)
     LEVEL_URI_SUFFIX = {0: ".abstract.md", 1: ".overview.md"}