Refactor SimHash banding logic into shared utility

[saurabhsharma2u]

Refactored the duplicated SimHash banding logic found in plugins/core/src/graph/cluster.ts and plugins/core/src/graph/duplicate.ts into a new static method SimHash.groupIntoBands in plugins/core/src/graph/simhash.ts. This improves code maintainability and reduces duplication. Confirmed that existing tests in clustering.test.ts and duplicate.test.ts pass.

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PR created automatically by Jules for task 6286707582081801071 started by @saurabhsharma2u

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[saurabhsharma2u]

Closing as this PR conflicts with the recent high-performance memory optimizations merged in PR 32. PR 32 rewrote to use flat Uint32Array/BigUint64Array typed arrays and index-based tracking, completely removing the Set<string> overhead. Introducing a generic groupIntoBands<T> that creates an array of Maps and Arrays of Objects would revert these critical performance wins. For cluster.ts, we might optimize it similarly in the future.