Remove the per-UDF Mutex serializing Rust UDF batches

Follow-up from review of #4459 ([thread](https://github.com/apache/datafusion-comet/pull/4459#discussion_r3712474949)).

`ImportedCScalarUdf` holds its kernel as `Mutex>`. Every `return_type` and `invoke_with_args` takes that lock, so all concurrent batches for a given UDF in a process serialize on one mutex, even though the kernel is logically immutable after load and the per-execution state already lives in a separate `CometCScalarKernelImpl` built fresh per call.

The lock is defensive rather than required: the comment notes DataFusion serializes invocations of a given `ScalarUDFImpl` anyway, and the FFI `Drop` is what is not `Sync`-safe.

@paleolimbot suggested `Arc` would maintain the reference counts and release the instance correctly when the last reference goes. Worth evaluating, along with simply holding `Box` directly given the struct is already `Send + Sync` and the kernel is only read after load.

Whatever shape this takes, it should preserve the property that the kernel's `release` runs exactly once, and it should not outlive the `LoadedLibrary` that dlopened it (see the field ordering in `LoadedLibrary`).

Read the ImportedCScalarUdf and CometCScalarKernel definitions, then inspect LoadedLibrary's field ordering and the existing review thread from #4459. Compare the Box and Arc ownership options, checking Send/Sync, FFI Drop behavior, and library lifetime. Done means concurrent batches no longer serialize on the per-UDF mutex, release runs exactly once, and the kernel cannot outlive its LoadedLibrary.

Written by the indexing model from the issue text.