updated benchmark plan

Author: joaoh82

docs/benchmarks-plan.md

@@ -277,7 +277,7 @@ Add the `duckdb-rs` driver under a `--features duckdb` flag. Wire only into Grou
 
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-## Open design questions
+## Decisions
 
 Same Q-list shape as the phase plans. Each one is "decide before sub-phase 9.1 starts."