Support descending range iteration in MapIndexStore

[hubgeter]

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  Support descending range iteration in MapIndexStore
  Add descending range iteration support to MapIndexStore by extending IndexMap with a descending range iterator and implementing it in IndexMapImpl. This makes MapIndexStore consistent with MemoryIndexStore for descending range queries and fixes the TODO fallback that previously used ascending iteration for a descending code path.
  Also add integration tests for descending range order, exclusive bounds, and end-to-end CompactRangeIndex query behavior with ORDER BY DESC and LIMIT.
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