[JingsongLi]

Purpose

Use Native execution for Ray fixed/POSTPONE input bucket routing and POSTPONE input statistics/inference, which previously still ran Python row hashing and sizing before the Native writers received input.

Changes

• Select a Native input router through dispatch before distributed tasks consume rows. Both Python and Native routers expose route(batch) -> Int32Array; Ray directly uses that interface, without method probing. Internal arguments consistently use router/bucket_router. Ray scheduling/grouping and public APIs remain unchanged.
• Keep Native adapters limited to Arrow/partition-plan transport and serializable configuration. The Native planner no longer inherits Python input statistics or inference; Rust Core owns these operations.
• Drop SDK objects during pickle and reconstruct from the shared schema/plan in workers. Runtime failures after backend selection propagate without replaying input through Python.
• Preserve positional statistics columns when partition keys are named row_count, data_size or total_buckets.
• Match current Java's NULL noncompact DECIMAL/TIMESTAMP/LTZ BinaryRow bytes, including decimal-overflow-to-NULL serialization, so Python hashing agrees with Native writers.
• Add backend-isolation, worker pickle, reordered/subset input, chunked/sliced Arrow, nullable FixedSizeBinary partition and exact Java hash regressions; update Ray docs.

Validation

• Focused Ray/router/dispatch regression suite: 49 passed, including the existing shuffle helper, chunked/empty inputs and both Python router modes.
• Serializer/routing/dispatch focused suites: 145 passed.
• Ray/postpone/table-write selection: 96 passed, 9 skipped.
• Joint current-SDK run: 139 passed, including the 8 new actual Native routing/planning cases, existing write/authorization tests and NULL-layout regressions; 36 Native plans exercised.
• Scoped flake8 and diff checks passed. No native extension was installed or replaced.

Dependency

Requires apache/paimon-rust#1207 and its BucketRouter and PostponeBucketPlanner SDK APIs. This PR is draft until that Rust change is merged. Native CI continues building Rust main; no SDK-version compatibility probes or fallback are introduced.

[leaves12138]

LGTM