Add partition-local exact semi/anti filter stage

[VibhuJawa]

Summary

Adds a focused PartitionedExactFilterStage for exact bucket-local exclusion or membership after two datasets have been partitioned with the existing ShuffleStage.

• Supports cuDF leftanti and leftsemi joins on one or more exact key columns.
• Matches each left shuffle task to the reference partition with the same partition_index.
• Preserves left-row multiplicity even when reference keys are duplicated.
• Validates complete-partition metadata and schemas before reading data.
• Rejects row-subsetting reads, multi-file writes, destructive path aliases, and missing reference partitions.
• Publishes per-partition input, reference, and output row counts in task metadata.

Why this is the missing primitive

Current Curator already provides the compatible hash shuffle. Campaign-specific grouping can be composed from that shuffle plus workload-specific reducers, but Curator does not currently provide the reusable bucket-local exact semi/anti operation needed for scalable exclusion without broadcasting the complete reference set. This PR deliberately adds only that missing reducer and no scheduler, checkpoint, or workload-specific control plane.

Composition

left ShuffleStage(key_fields, N) ---------+
                                           +--> PartitionedExactFilterStage(mode="leftanti" | "leftsemi")
reference ShuffleStage(key_fields, N) ----+

Both shuffles must use identical keys, hashing, and partition count.

Validation

• 37 focused CPU tests passed; 2 GPU semantics cases are separately marked.
• git diff --check passed.
• The three-file diff is compared against exact NVIDIA main commit 6fa52e3e597206849acb2ac119413fa6cbfe644d through non-default review base sync/nvidia-main-6fa52e3e; the stale fork default main was not modified.