Fix num-bigint-dig future-incompat warning

[beinan]

Summary
cargo check --workspace reports num-bigint-dig v0.8.4 as future-incompatible with a coming Rust release. We should update or patch to avoid upcoming breakage.

Repro

cargo check --workspace

Expected
No future-incompatibility warnings.

Notes

• Identify the dependency chain pulling num-bigint-dig (likely via crypto deps).
• Try cargo update -p num-bigint-dig once a fixed release exists, or add a [patch.crates-io] override if needed.

Environment

• Rust workspace after moving Python bindings to crates/lance-graph-python
• Rust toolchain from CI defaults