* [BugFix] Guard replicated reducer updates
Use only the canonical T.Parallel replica when updating fully replicated reducer buffers, preventing over-counting across differently replicated fragment layouts. Add short-extent and mixed-extent numerical regressions.
* [Refactor] Use ordered reducer buffer lookup
Keep the small reducer buffer collection as an ffi::Array and perform identity membership checks with Buffer::same_as, avoiding unordered STL in loop partitioning.
* [Refactor] Clarify replicated reducer guard naming
* [Docs] Mark reducer partition hook as temporary
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Co-authored-by: LeiWang1999 <leiwang1999@outlook.com>