More index information in snapshot

Store basic index parameters to reduce construction overhead of different indices.

For example, by just storing the lower and upper bound values of a numeric range tree, we can deduce the correct tree beforehand and avoid splitting/reallocating nodes.

The issue names no files or tests. Start by tracing snapshot index construction and numeric range tree creation, then determine which basic bounds can be retained to select the correct tree without unnecessary splitting or node reallocation. Done means the snapshot stores the needed parameters and index construction avoids the stated overhead without changing behavior.

Written by the indexing model from the issue text.