Add docs sidecar for quasi10B sp1024 export

datasets/docs_selected.source_manifest.json

@@ -0,0 +1,12 @@
+{
+  "source_export_root": "/root/exports/fineweb_50Bsub100B_50keval_v0",
+  "snapshot_kind": "partial_docs_cache_from_50B_export",
+  "note": "not canonical 10B shard selection; train split is a paused snapshot of the 50B shuffled train stream",
+  "selection_seed": 1337,
+  "num_val_docs": 50000,
+  "num_docs": 15368808,
+  "docs_val": 50000,
+  "docs_train": 15318808,
+  "docs_bytes": 48166275520,
+  "docs_sha256": "84386dfa7b339a5d4831d5273c4a2028b78b60670d3a235633a8520545d19bc7"
+}