feat: add pk-index compaction resolver and apply stream operators

[silver-ymz]

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What's changed and what's your intention?

Stack: PR 3 of 6 for iceberg pk-index compaction (← #26563 | → #26565)

Adds two new stream executors that form the core pk-index compaction operator pipeline, plus the proto definitions they need.

• CompactionResolverExecutor: resolves which position delete files to compact based on delete file count thresholds and partition grouping
• CompactionApplyExecutor: receives compaction task outputs and rewrites data files by merging position deletes into new data files
• from_proto/compaction_resolver.rs: proto deserialization for the new executor
• proto/stream_plan.proto: add IcebergPkIndexBarrierMutation and new stream node types
• proto/stream_service.proto: add PB_ICEBERG_PK_INDEX_SINK_ROLE_COMPACTION_RESOLVER
• executor/mod.rs: register new NodeBody variants. IcebergPkIndexBarrierMutation is used for the next PR.

The executors are defined but not yet wired into the task system — PR 4 does that.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

[silver-ymz]

• test: add E2E and simulation tests for pk-index compaction #26567
• feat: coordinate pk-index compaction via barrier checkpoint and commit #26566
• feat: wire pk-index compaction to stream task and iceberg compactor #26565
• feat: add pk-index compaction resolver and apply stream operators #26564 👈 (View in Graphite)
• refactor: extract position delete helpers and simplify pk-index writer #26563
• refactor: introduce IcebergCompactionTaskId type #26562
• main

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