[VL] Subdivide SplitRV CPU/wall timer into 4 sub-stages

[luis4a0]

What changes are proposed in this pull request?

Companion follow-up to #12083 (which made cpuWallTimingList_ actually record real numbers) and #12107 (which added a per-batch input-encoding counter). The existing CpuWallTimingSplitRV bucket wraps the entire splitRowVector() call, so when it dominates wall time in a VELOX_SHUFFLE_WRITER_LOG_FLAG=1 profile, there's no way to tell which of the four sub-paths is responsible: fixed-width scatter, validity copy, binary split, or complex (Presto-serialized) split.

This PR subdivides SplitRV into four new buckets, one per helper inside splitRowVector():

New enum	Helper
CpuWallTimingSplitFixedWidth	splitFixedWidthValueBuffer()
CpuWallTimingSplitValidity	splitValidityBuffer()
CpuWallTimingSplitBinary	splitBinaryArray()
CpuWallTimingSplitComplex	splitComplexType()

The outer SplitRV bucket is unchanged: it continues to count once per call, and its wall/cpu measurement now happens to overlap the four inner buckets (the outer timer is still ticking while each inner one is). The outer minus the sum-of-inners is therefore the time spent in splitRowVector() outside the four sub-paths (mostly the post-split partitionBufferBase_ update loop). This is the same parent/child pattern most profiling tools use for nested timers, and it's documented in a comment next to the enum.

The CpuWallTimingType enum, the CpuWallTimingName() helper, and a new single-value accessor cpuWallTiming(CpuWallTimingType) are hoisted from protected to public so the data is accessible from tests. The underlying cpuWallTimingList_ storage stays protected.

How was this patch tested?

This PR adds cpp/velox/tests/VeloxHashShuffleWriterSplitRvSubStagesTest.cc (second commit) with five gtest cases:

• enumNames: validates the strings emitted under VELOX_SHUFFLE_WRITER_LOG_FLAG for the 4 new enum entries (these strings are stable observable surface).
• fixedWidthBumpsItsBucket: single fixed-width batch; asserts each of the 4 inner buckets has count == 1 (every helper inside splitRowVector() is called once per batch regardless of column-type mix, since the new sub-stage SCOPED_TIMER calls live at function entry).
• binaryBumpsItsBucket: VARCHAR batch; asserts the binary sub-stage's wallNanos > 0, confirming real work is being measured (not just function-entry overhead).
• complexBumpsItsBucket: ARRAY batch; same wallNanos > 0 assertion against the complex sub-stage (Presto-serialized round-trip per partition).
• outerSplitRvStillCounted: sanity check, two batches in and outer SplitRV bucket has count == 2. Confirms the new inner timers refine, not replace, the existing outer timer.

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