Follow-up to #4.

#4 generates Java classes for the datafusion-proto schema. This issue is the end-to-end wiring that makes those classes useful — Java code constructs a LogicalPlanNode, hands it to SessionContext, and gets back a DataFrame that streams Arrow batches the same way ctx.sql(...) does today.

Sketch

Three pieces sit between Java-built protobuf and execution:

1. Java surface. A new method on SessionContext:

   public DataFrame fromProto(byte[] planBytes);

   No other Java work is needed — the protobuf builders generated by Add datafusion-proto build #4 are the public construction API.

2. JNI bridge. A single new native method that takes the byte array and returns a DataFrame pointer, modeled on the existing sql() JNI shim. Arrow FFI stays on the result path; the plan crosses JNI as a primitive byte[].

3. Rust deserialization + execution. Add datafusion-proto = \"53\" to native/Cargo.toml, then in the JNI implementation:

   let node = datafusion_proto::protobuf::LogicalPlanNode::decode(&bytes[..])?;
   let plan = node.try_into_logical_plan(&ctx.state(), &DefaultLogicalExtensionCodec {})?;
   let df = runtime.block_on(ctx.execute_logical_plan(plan))?;

   Wrap the resulting DataFrame the same way sql() does and return its pointer.

Open design questions for this issue

• Logical vs. physical plan first. Recommend logical so DataFusion's own optimizer runs; physical is a follow-up.
• Schema discovery. A JVM caller building a TableScanNode needs the schema of registered tables. Likely a new JNI shim like SessionContext.tableSchema(name) -> org.apache.arrow.vector.types.pojo.Schema. Without this, the API is not usable for anything but trivial plans.
• Extension codec. DefaultLogicalExtensionCodec is fine for now; a real codec arrives with Java-defined UDFs (the fourth roadmap item).
• Round-trip tests. Build a plan in Java, execute it, and compare results against the same query run via ctx.sql(...) over the TPC-H integration data.

Out of scope here

• Java-defined UDFs and their custom extension codec.
• Physical plan submission.
• Any builder helpers / fluent API on top of the raw protobuf builders.