Deduplicate byte-identical documents (per chunk size)

Collapses source_urls that are byte-identical full documents to a single canonical copy per documents-{N} config, and remaps query gold source_urls to the canonical (gold char spans unchanged). Makes nDCG@10 well defined (only one correct chunk per query).

queries/test-00000-of-00001.parquet

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:e76b9ecce331969f8b2c45d6bc52d3a38667315158d16546bbbdefa72910708a
-size 509913
+oid sha256:0c70dd6cdf2a587638c2cdf781df44779bd971e723bc07b8fab6526868ef3890
+size 548597