feat(java): support distributed splits planning

[summaryzb]

Summary

This PR adds distributed split planning for Lance's filtered read execution, enabling a plan/execute separation pattern where a coordinator node plans a scan and worker nodes execute per-fragment portions of it. The implementation spans the Rust core (scanner, filtered read exec, protobuf serialization) and Java bindings, with a new FilteredRead Java class that exposes the full workflow.

Problem

Lance's FilteredReadExec supports serializing an entire scan plan to protobuf for remote execution, but there was no mechanism to split a multi-fragment plan into per-fragment tasks that could be distributed to individual workers. Distributed engines like Spark need to: (1) plan a scan on the coordinator, (2) split the plan into independent per-fragment tasks, (3) serialize each task and ship it to a worker, and (4) execute each task independently. The existing code could serialize and deserialize a full plan, but lacked the splitting, metadata extraction, and end-to-end orchestration API needed for this workflow.

Approach

The implementation follows a three-layer design:

Rust core -- Scanner entry point (scanner.rs): A new Scanner::plan_filtered_read() method constructs a FilteredReadExec from the current scanner settings (filter, projection, fragments, batch size, etc.), triggers internal planning via ensure_plan_initialized() to compute the RowAddrTreeMap (which fragments/rows to read), and serializes the result to protobuf bytes. This mirrors the logic in create_plan / new_filtered_read but exposes the result as an opaque serializable blob rather than executing it.

Rust core -- Proto splitting and execution (filtered_read_proto.rs): Three new public functions:

• split_plan_proto() decodes a full FilteredReadExecProto, iterates over the RowAddrTreeMap to extract per-fragment entries, and re-serializes each as a standalone proto with the same table identifier and options but a single-fragment plan. The global scan_range_after_filter is intentionally dropped from per-fragment protos since it can only be applied after aggregating results across all workers.
• extract_plan_metadata() provides a lightweight summary (fragment IDs and row counts per fragment, where -1 means "full fragment") without needing a dataset handle, enabling coordinators to estimate task sizes for load balancing.
• split_and_inspect_plan_proto() combines both operations in a single decode pass to avoid redundant deserialization.
• execute_filtered_read_from_bytes() is the worker-side counterpart: it decodes a proto, reconstructs a FilteredReadExec (optionally reusing an existing dataset handle), and returns a SendableRecordBatchStream.

Rust core -- FilteredReadExec (filtered_read.rs): A new ensure_plan_initialized() method triggers internal plan computation without converting to the external FilteredReadPlan format, caching the result for subsequent serialization.

Java bindings: A new FilteredRead class implements Serializable and provides the full distributed workflow API:

• planFilteredRead(scanner) calls through JNI to Scanner::plan_filtered_read() and split_and_inspect_plan_proto(), returning a FilteredRead object containing the full proto, per-fragment split protos, fragment IDs, and rows-per-fragment metadata.
• getTasks() returns the per-fragment task protos for distribution.
• executeFilteredRead(dataset, taskProto, allocator) executes a single task on a worker node.
• Split protos and metadata arrays are marked transient -- they are not included in Java serialization; the receiver reconstructs them by calling split_and_inspect_plan_proto on the deserialized full proto.

Per-fragment filter deduplication from the original plan is preserved: split_plan_proto looks up each fragment's filter expression ID in the shared filter_expressions array and copies only the relevant entry into the per-fragment proto.

Changes

Rust -- rust/lance/src/dataset/scanner.rs:

• Added Scanner::plan_filtered_read() method (gated on substrait feature) that builds a FilteredReadExec, triggers planning, and serializes to protobuf bytes.

Rust -- rust/lance/src/io/exec/filtered_read.rs:

• Added FilteredReadExec::ensure_plan_initialized() public method to trigger and cache plan computation without external conversion.

Rust -- rust/lance/src/io/exec/filtered_read_proto.rs:

• Added FilteredReadPlanMetadata struct and extract_plan_metadata() function for lightweight plan inspection.
• Added execute_filtered_read_from_bytes() for worker-side deserialization and execution.
• Added split_plan_proto() for per-fragment plan splitting.
• Added SplitPlanResult struct and split_and_inspect_plan_proto() for combined split + metadata extraction.

Proto -- protos/filtered_read.proto:

• Added a clarifying comment to FilteredReadPlanProto.row_addr_tree_map field.

Java JNI -- java/lance-jni/src/blocking_scanner.rs:

• Added nativeCreatePlan and nativeExecuteFilteredRead JNI entry points.

Java -- java/src/main/java/org/lance/ipc/FilteredRead.java:

• New FilteredRead class implementing Serializable with full distributed workflow API.

Java -- java/src/test/java/org/lance/FilteredReadTest.java:

• New end-to-end test class for the distributed filtered read workflow.

Test Coverage

• Rust -- test_extract_plan_metadata: Verifies fragment IDs and row counts are correctly extracted from a serialized plan (2 fragments, 50 rows each).
• Rust -- test_split_plan_proto: Splits a 2-fragment plan and verifies each split executes independently, with total rows matching direct execution.
• Rust -- test_split_plan_proto_with_filter: Same as above but with a filter expression (x > 10), verifying per-fragment filters are correctly preserved in splits.
• Rust -- test_plan_serialize_execute_roundtrip: End-to-end: build exec with filter, plan, serialize to bytes, deserialize via execute_filtered_read_from_bytes, and verify identical output.
• Java -- testBasicPlanAndExecute: Plans a 2-fragment dataset, splits into tasks, executes each, and verifies total row count (50 rows).
• Java -- testPlanMetadata: Verifies fragment count, fragment IDs, and rows-per-fragment arrays for a 3-fragment dataset.
• Java -- testDistributedSplitAndExecute: Simulates coordinator/worker pattern with 3 fragments, compares distributed execution total with direct scan total (60 rows).
• Java -- testPlanWithFilter: Plans with id > 10 filter on a 40-row fragment, verifies 29 rows returned after distributed execution.
• Java -- testSerializableRoundtrip: Serializes a task proto via Java ObjectOutputStream, deserializes, executes both, and verifies identical row counts.