Document native UDF ABI alignment requirements

[nvliyuan]

Summary

• Document that native UDF builds must align cuDF/RMM/CCCL headers with the libcudf.so packaged in the rapids-4-spark jar.
• Call out undefined symbols and native crashes as symptoms of ABI drift, especially for snapshot or locally built jars.
• Recommend rebuilding native UDFs after jar changes and removing cached native dependencies before rebuilds.

Closes #628

Test plan

• Documentation-only change; verified README diff locally.
• Checked Cursor lints for the edited README.

[greptile-apps]

Greptile Summary

This PR adds a "Native ABI compatibility note" paragraph to the fast-build section of the UDF README, documenting that native UDFs must be compiled against cuDF/RMM/CCCL headers that match the libcudf.so bundled in the rapids-4-spark jar, and prescribing concrete remediation steps.

• Documents the ABI drift failure modes (undefined symbols, native crashes) that arise when headers used at compile time do not match the jar's libcudf.so, with extra guidance for snapshot and locally built jars.
• Instructs users to remove target/native-deps and target/cudf-repo before rebuilding after any version or branch change, preventing stale headers or cached libraries from silently causing mismatches.

Confidence Score: 5/5

Documentation-only change; no runtime code is modified, so there is no risk of behavioral regression.

The change is a single prose addition to a README with no code, build scripts, or configuration files touched. The new text is technically accurate about ABI drift, names the correct cache directories to clear, and is placed logically within the existing fast-build section.

No files require special attention.

Important Files Changed

Filename	Overview
examples/UDF-Examples/RAPIDS-accelerated-UDFs/README.md	Adds an 18-line 'Native ABI compatibility note' section explaining header/library alignment requirements and cache-clearing steps for native UDF builds.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Change rapids-4-spark jar version] --> B{Headers match libcudf.so in jar?}
    B -- Yes --> C[Build succeeds, UDF loads correctly]
    B -- No --> D[ABI drift: undefined symbols or native crash]
    A --> E[Update cudf.git.branch, rapids-cmake branch, RMM/CCCL versions]
    E --> F[Remove target/native-deps and target/cudf-repo]
    F --> G[mvn clean package -Pudf-native-examples]
    G --> B

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[thirtiseven]

Thanks!