Enable Cache Reuse for ExtMemQuantileDMatrix Across Sessions

Currently, `ExtMemQuantileDMatrix` creates cache files with names that include the memory address of the `DMatrix` object (e.g., cache-0x7f8b4c000000.gradient_index.page). This makes it impossible to reuse these cache files across different process sessions, even when working with the exact same data.

Although I understand that this design feature was on purpose, it would be good if we can enable something similar as it would save a large amount of pre-computing the same data if we could reuse the `.page` files generated, as long as we know they are being generated from the same input DataIter.

Start by tracing ExtMemQuantileDMatrix cache-file naming and the DataIter input lifecycle. Determine how cache files can safely identify identical input across process sessions; done means .page files are reused only when they came from the same input, with coverage for cross-session reuse.

Written by the indexing model from the issue text.