Optimize Parallel instance in GenSpawnInstances to avoid allocation.

[nmichael44]

Motivation

Currently in GenSpawnInstances, the Parallel instance provides .parallel and .sequential as defs that instantiate a new (M ~> F) and new (F ~> M) on every invocation.

Because operations like cats.Parallel.parTraverse invoke P.parallel(ma) and P.sequential(ma) for every item in the collection, this creates a large number of FunctionK object allocations during a traversal, putting unnecessary pressure on the garbage collector.

Changes

This PR eliminates these allocations by applying the Id caching technique (similar to Resource.liftK).

• Extracts the FunctionK instances into a private object GenSpawnInstances to act as true JVM-wide singletons (rather than trait fields).
• Uses asInstanceOf to cast the cached Id transformations to the required effect type at the call site.

Impact

Zero-allocation parallel and sequential transformations for all effect types that implement GenSpawn. This reduces the memory footprint and GC overhead on hot paths involving parallel collections processing.