Consolidate date/timestamp truncation and xxhash64 with upstream DataFusion implementations #5103

What is the problem the feature request solves?

Beyond arrow kernels, two areas duplicate functionality now available upstream in DataFusion / datafusion-spark (54.1):

1. Date/timestamp truncation: native/spark-expr/src/kernels/temporal.rs implements trunc(date, fmt) and date_trunc(fmt, ts) by hand. Arrow has no truncation kernel, but datafusion_functions::datetime::date_trunc is timezone-aware (chrono-tz), supports all the granularities Spark needs with Monday-based weeks, and preserves the input tz annotation. Upstream datafusion-spark's trunc already composes it with the same alias map (yy/yyyy to year, mm/mon to month) via a cast sandwich.
2. xxhash64: datafusion-spark 54.1 ships a Spark-compatible SparkXxhash64 (twox-hash based, with its own type-dispatch machinery). Comet's hash_funcs/xxhash64.rs could potentially be rewired to it. Note datafusion-spark has no murmur3, so Comet's murmur3.rs stays.

Describe the potential solution

• Truncation: wire date_trunc at the physical level for the scalar-format paths (datafusion-spark's trunc/date_diff are simplify-only UDFs whose invoke_with_args returns an internal error, so they cannot be called from Comet's physical-plan pipeline directly). Keep custom code for: per-row-format variants (DataFusion requires scalar granularity), dictionary inputs, and the Spark session-timezone pre-shift.
• Requires differential DST-boundary testing first: Comet resolves ambiguous local times to the earliest occurrence and falls back to the original instant on gaps (test_timestamp_trunc_dst_boundary); DataFusion's fine-versus-coarse granularity handling needs case-by-case verification.
• xxhash64: verify type coverage before rewiring (Comet's macro additionally handles the small/large Decimal128 split, Map, FixedSizeList, and Struct fast paths).

Additional context

Related in spirit: string_funcs/contains.rs and array_funcs/array_slice.rs each document an upstream DataFusion issue that, once fixed, lets the Comet file be dropped entirely.

Found during an audit of native code that replicates existing arrow-rs kernels.