Expose a named variant of `join_core` (mirroring `arrange_named`) for operator-level profiling

Hi all, I noticed that arrangements can be given custom operator names via arrange_named / arrange_core(name), which is really helpful for identifying them in logging and profiling streams.

Joins don’t seem to have an equivalent right now: join_core hardcodes the operator name as "Join" (operators/join.rs:81), so every join shows up with the same name in OperatesEvent, making it hard to distinguish them in dataflows with many joins.

Would you be open to adding a named variant, e.g. join_core_named(other, name, logic), with join_core delegating to it using "Join"? It seems like a small backward-compatible change that mirrors arrange_named, and it would make profiling/debugging much easier.

Start with operators/join.rs:81 and compare its join_core implementation with arrange_named and arrange_core(name). Add the named join entry point while preserving join_core's default "Join" label, then verify that joins report distinct names in OperatesEvent and that existing behavior remains unchanged.