refactor(spark): Extract XxHash64 helper

[malinjawi]

Summary:

• Move the Spark-compatible XxHash64 helper out of Hash.cpp into
  XxHash64.h so Spark aggregate code can reuse it.
• Keep the existing hash implementation behavior unchanged. This is intended
  as a pure refactor.
• Add the new header to the Spark functions CMake target.

Why:

• Split the reusable hashing refactor out of feat(spark): Add approx_count_distinct_for_intervals aggregate function #16595 as requested by reviewers.
• Keep the follow-up aggregate PR focused on aggregate behavior only.

Review notes:

• This is PR 1/3 in the split stack.
• Follow-up PRs: feat(exec): Forward constant inputs in SimpleAggregateAdapter #17682 and feat(spark): Add approx_count_distinct_for_intervals aggregate #17683.

Testing:

• git diff --check upstream/main..pr16595-xxhash64-refactor
• SKIP=action-validator pre-commit run --from-ref upstream/main --to-ref pr16595-approx-count-distinct-intervals

Part of #16595.

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[github-actions]

Build Impact Analysis

Selective Build Targets (building these covers all 20 affected)

cmake --build _build/release --target spark_expression_fuzzer_test velox_date_extract_benchmark velox_expression_runner_test velox_expression_runner_unit_test velox_functions_spark_aggregates_test velox_functions_spark_test velox_spark_function_registry_test velox_spark_query_runner_test velox_sparksql_benchmarks_cast velox_sparksql_benchmarks_compare velox_sparksql_benchmarks_from_json velox_sparksql_benchmarks_get_funcs velox_sparksql_benchmarks_hash velox_sparksql_benchmarks_in velox_sparksql_benchmarks_simd_compare velox_sparksql_benchmarks_split velox_sparksql_coverage

Total affected: 20/580 targets

Warning: 1 file(s) could not be mapped to any target. A full build may be needed.

• velox/functions/sparksql/CMakeLists.txt

Affected targets (20)

Directly changed (1)

Target	Changed Files
velox_functions_spark_impl	Hash.cpp, XxHash64.h

Transitively affected (19)

• spark_expression_fuzzer_test
• velox_date_extract_benchmark
• velox_expression_runner
• velox_expression_runner_test
• velox_expression_runner_unit_test
• velox_functions_spark
• velox_functions_spark_aggregates_test
• velox_functions_spark_test
• velox_spark_function_registry_test
• velox_spark_query_runner_test
• velox_sparksql_benchmarks_cast
• velox_sparksql_benchmarks_compare
• velox_sparksql_benchmarks_from_json
• velox_sparksql_benchmarks_get_funcs
• velox_sparksql_benchmarks_hash
• velox_sparksql_benchmarks_in
• velox_sparksql_benchmarks_simd_compare
• velox_sparksql_benchmarks_split
• velox_sparksql_coverage

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Slow path • Graph generated from PR branch