[EPIC] Iceberg Feature Matrix: Spark-Iceberg vs iceberg-rust vs datafusion-comet

We should use below matrix to check for any missing implementations that could accelerate Spark Iceberg pipeline using comet

### READ PATH

| Feature | Iceberg Java | iceberg-rust | datafusion-comet (via iceberg-rust) |
|---|---|---|---|
| Basic Parquet scan | Yes | Yes | Yes - IcebergScanExec |
| Positional deletes (V2 MoR) | Yes | Yes - ArrowReader + DeleteVector (RoaringTreemap) | Yes - delegates to ArrowReader with row_selection_enabled(true) |
| Equality deletes (V2 MoR) | Yes | Yes - ArrowReader builds equality delete predicates | Yes - delegates to ArrowReader |
| Deletion vectors (V3) | Yes - DVUtil, DVFileWriter, DVIterator | Yes - DeleteVector + Puffin deletion-vector-v1 blob support | Not wired - Comet doesn't pass DV metadata via protobuf |
| Schema evolution | Yes | Yes | Yes - IcebergStreamWrapper adapts batches to target schema |
| Partition pruning (static) | Yes | Yes | Yes - partitions serialized in protobuf |
| Dynamic partition pruning | Yes (Spark) | N/A (engine-level) | Yes - CometIcebergNativeScanExec defers serialization for DPP |
| Row-group filtering (residuals) | Yes | Yes | Yes - residual predicates converted to iceberg::expr::BoundPredicate |
| Identity partition columns | Yes | Yes | Yes |
| Object stores (S3/GCS/OSS) | Yes (Hadoop FS) | Yes (OpenDAL) | Yes (OpenDAL via FileIOBuilder) |
| V1 spec | Yes | Yes | Yes |
| V2 spec | Yes | Yes | Yes |
| V3 spec metadata | Yes | Yes (FormatVersion::V3, next_row_id, row lineage) | Not used - Comet doesn't handle V3-specific metadata |

### WRITE PATH

| Feature | Iceberg Java | iceberg-rust | datafusion-comet |
|---|---|---|---|
| Data file writing | Yes - DataWriter | Yes - DataFileWriter | No - uses raw parquet crate, not iceberg-rust |
| Partitioned writes (sorted) | Yes - ClusteredDataWriter | Yes - ClusteredWriter | No - writes single file per Spark partition |
| Partitioned writes (fanout) | Yes - FanoutDataWriter | Yes - FanoutWriter | No |
| Rolling file writer | Yes | Yes - RollingFileWriter | No |
| Equality delete writer | Yes | Yes - EqualityDeleteWriter | No |
| Position delete writer | Yes | Partial | No |
| Deletion vector writer | Yes - DVFileWriter, PartitioningDVWriter | No explicit DV writer | No |
| AppendFiles / FastAppend | Yes - AppendFiles | Yes - FastAppendAction | No - commit done in Java |
| OverwriteFiles | Yes - OverwriteFiles | Missing | No |
| ReplacePartitions | Yes - ReplacePartitions | Missing | No |
| DeleteFiles | Yes - DeleteFiles | Missing | No |
| RowDelta | Yes - RowDelta | Missing | No |
| RewriteFiles | Yes - RewriteFiles | Missing | No |
| Transaction + commit | Yes - full atomic commit | Yes - Transaction::commit() with retry | No - commit is JVM-side |

### ROW-LEVEL OPERATIONS (DELETE/UPDATE/MERGE)

| Feature | Iceberg Java + Spark | iceberg-rust | datafusion-comet |
|---|---|---|---|
| Copy-on-Write (CoW) scan | Yes - SparkCopyOnWriteScan | No CoW scan | No |
| Copy-on-Write write | Yes - rewrite affected data files | Partial (rewrite manually) | No |
| Merge-on-Read (MoR) scan | Yes - buildMergeOnReadScan() | Yes - ArrowReader applies deletes | Yes (read only) |
| MoR position delta write | Yes - SparkPositionDeltaWrite | No | No |
| DELETE FROM | Yes (CoW or MoR) | No action | No |
| UPDATE | Yes (CoW or MoR) | No action | No |
| MERGE INTO | Yes - SparkRowLevelOperationBuilder | No (issue #2201) | No