fix(java): expose updatedFragmentOffsets on Update operation for RewriteColumns

[jerryjch]

Summary

• Follow-up to fix: propagate update_columns offsets and partial last_updated for RewriteColumns #6650 (fix: propagate update_columns offsets and partial last_updated for
  RewriteColumns). Required by lance-spark#418.
• Update.java: adds Map<Long, byte[]> updatedFragmentOffsets field, 7-arg constructor,
  accessor updatedFragmentOffsets(), and Builder.updatedFragmentOffsets(...) setter.
  Defaults to Collections.emptyMap(). Values are portable RoaringBitmap bytes.
• java/lance-jni/src/transaction.rs — two JNI directions updated: FromJava deserializes
  each byte[] value into a RoaringBitmap and sets updated_fragment_offsets on the Rust
  operation; IntoJava serializes each bitmap to byte[] and populates a HashMap<Long, byte[]>
  passed to the 7-arg Update constructor (previously the field was ignored and the 6-arg
  form was used).
• Update.java equals / hashCode: deep-compares byte[] values by content; hashCode
  added per the Java contract.

Background

PR #6650 added updated_fragment_offsets on the Rust Operation::Update (proto field 9),
build_manifest partial refresh logic, and FragmentUpdateResult.getUpdatedRowOffsets().
Two gaps remained:

1. The Java Update class had no field for these offsets and convert_to_rust_operation
   always set updated_fragment_offsets: None, so the lance-spark commit path
   (UpdateColumnsBackfillBatchWrite) had no way to pass offsets to Rust and the partial
   refresh in build_manifest could never activate from a JVM caller.

2. convert_to_java_operation_inner still used the old 6-arg constructor signature for
   new_object. With the 6-arg constructor removed from Update.java (replaced by the
   7-arg form), any Rust→Java materialization of Operation::Update (e.g. reading back a
   transaction) would fail at runtime with NoSuchMethodError.

Implementation notes

• Values are portable RoaringBitmap bytes (little-endian, spec-compliant). The JNI boundary
  stays O(bitmap size) rather than O(n matched rows).
• with_local_frame(4, ..) per bitmap entry in IntoJava bounds local-ref growth on large
  offset maps. JMap was avoided inside the frame because it holds a JObject with the
  outer frame's lifetime, causing borrow-checker conflicts; call_method on the outer
  java_map reference is used instead.
• The Vec<u8> buffer for each bitmap is allocated in Rust before entering the frame, so
  its lifetime is independent of JNI frame scope.
• UpdatedFragmentOffsets added to the lance::dataset::transaction import.

Why the protobuf field alone is not enough

lance-spark commits by calling CommitBuilder.execute(transaction), which passes the Java
Transaction object to nativeCommitToDataset via JNI. The JNI handler calls
convert_to_rust_transaction → convert_to_rust_operation, which reflects on the Java
Update object to build the Rust Operation::Update struct. The protobuf field (field 9)
is only used when a Transaction is serialized as a proto blob; it has no effect on the
reflection-based JNI path unless the Java Update class exposes the field and the JNI
deserialization reads it.

Test plan

• UpdateTest#testUpdatedFragmentOffsetsRoundTrip — commits an Update with a non-empty
  updatedFragmentOffsets map through CommitBuilder.execute (exercises the FromJava JNI
  path), reads the transaction back via Dataset.readTransaction() (exercises the IntoJava
  JNI path), and asserts the offsets match. Map value is hardcoded portable RoaringBitmap
  bytes encoding {1, 3, 5}; verified with assertArrayEquals after the round-trip.

Compatibility

• Additive new API on Update — the updatedFragmentOffsets field did not exist in any
  prior release. The builder setter is optional and defaults to Collections.emptyMap(), so
  existing Update.builder()...build() call sites compile and behave identically.
• Java — equals / hashCode: equals uses offsetMapsEqual to deep-compare byte[]
  values via Arrays.equals; hashCode is added per the Java contract.
• JNI — constructor signature: the IntoJava new_object call is updated from the 6-arg to
  the 7-arg form in the same PR. Both files must ship together; within that atomic change
  there is no compatibility gap.
• Rust / proto: no changes. The updated_fragment_offsets proto field and Rust struct
  field were already added in fix: propagate update_columns offsets and partial last_updated for RewriteColumns #6650.

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[jerryjch]

cc @jackye1995 @hamersaw