feat(java): expose mergeInsertUncommitted (execute_uncommitted) in Java SDK

Problem

The Java SDK's Dataset.mergeInsert() always commits immediately. There is no way to perform a merge-insert and hold the resulting Transaction for manual commit via CommitBuilder.

Rust core already supports this via MergeInsertJob::execute_uncommitted() and MergeInsertJob::execute_uncommitted_batches(), and the Python SDK already exposes MergeInsertBuilder.execute_uncommitted() and execute_uncommitted_batches().

Use Case

Callers need to inspect or mutate the uncommitted Transaction (e.g., add extra operations, validate before commit) before finalizing, rather than having the merge-insert auto-commit.

Current State

What's Missing

All the building blocks are already in place — CommitBuilder, Transaction, MergeInsertParams, MergeInsertStats all exist in Java. The gap is small:

1. JNI (java/lance-jni/src/merge_insert.rs): New JNI function calling execute_uncommitted instead of execute_reader, returning (Transaction, MergeStats) across the JNI boundary.

2. Java (java/src/main/java/org/lance/):

   • New UncommittedMergeInsertResult POJO holding Dataset, Transaction, MergeInsertStats
   • New Dataset.mergeInsertUncommitted(MergeInsertParams, ArrowArrayStream) method
   • Corresponding native declaration

Estimated total: ~150 lines of binding code. No new Rust core logic needed.

Proposed API

UncommittedMergeInsertResult result = dataset.mergeInsertUncommitted(
    new MergeInsertParams(List.of("id"))
        .withMatchedUpdateAll()
        .withNotMatched(WhenNotMatched.InsertAll),
    sourceStream
);

// Inspect / modify transaction before commit
Transaction txn = result.transaction();
Dataset committed = new CommitBuilder(result.dataset()).execute(txn);

References

• Rust: rust/lance/src/dataset/write/merge_insert.rs — execute_uncommitted() / execute_uncommitted_batches()
• Python binding: python/src/dataset.rs — MergeInsertBuilder::execute_uncommitted() (line 468)
• Existing Java mergeInsert: java/src/main/java/org/lance/Dataset.java (line 2154)
• Existing Java JNI: java/lance-jni/src/merge_insert.rs

Start with java/src/main/java/org/lance/Dataset.java and java/lance-jni/src/merge_insert.rs, then compare the Rust execute_uncommitted APIs with the Python binding in python/src/dataset.rs. Done means the Java SDK exposes the uncommitted result, its native declaration, and JNI support for returning the Transaction and merge statistics so callers can commit manually.