feat: add deletion vector compaction planner

Add a snapshot-bound metadata-only planner that selects rewrite candidates by file size and deduplicated file-scoped position-delete pressure. Validate planner inputs and group candidates deterministically within partitions using target-sized groups.

Author: wirybeaver

src/iceberg/CMakeLists.txt

@@ -26,6 +26,7 @@ set(ICEBERG_SOURCES
     catalog/memory/in_memory_catalog.cc
     catalog/session_catalog.cc
     catalog/session_context.cc
+    compaction_planner.cc
     delete_file_index.cc
     deletes/dv_util.cc
     deletes/dv_writer.cc