In src/mongo/bson/bsoncolumn_interleaved.h, struct `Decoder64` (nested within `BlockBasedInterleavedDecompress::DecodingState`) has a default constructor `Decoder64() = default;` which does not initialize `scaleIndex` (a raw `uint8_t`), which has no default member initializer. A default-constructed instance therefore holds an indeterminate `scaleIndex`.

Reading the indeterminate value before explicit assignment constitutes undefined behavior. While the practical impact may be limited if the member is always set before use, the uninitialized read remains UB and should be fixed to ensure code correctness and eliminate static analysis warnings.

Found by static analysis (Svace, UNINIT.CTOR): "Constructor declared at bsoncolumn_interleaved.h:233 may not initialize class members of 'mongo::bsoncolumn::BlockBasedInterleavedDecompress::DecodingState::Decoder64'. Following members aren't initialized: scaleIndex."

Suggested fix: initialize `scaleIndex` in the default constructor or add a default member initializer (e.g., `uint8_t scaleIndex = 0;`).