[BugFix] Deduplicate replicated reducer updates (#2881)

* [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>