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

[luis4a0]

What changes are proposed in this pull request?

Stacked on top of #12107. Until that PR merges this PR's diff will appear larger (it carries both #12107's commits and this PR's commits). Once #12107 lands in main, the diff here will narrow to just the SplitRV sub-stage changes (2 commits, ~232 lines). Reviewers may wait for #12107 to merge if they prefer to look at this PR's contribution in isolation.

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.

To make the new sub-stage data accessible to anything other than the existing compile-time log path (tests, future ShuffleWriterMetrics exposure, profilers), three things are hoisted from protected to public in VeloxShuffleWriter:

• The CpuWallTimingType enum
• The CpuWallTimingName() helper (already de-facto public, since its output is printed by the VELOX_SHUFFLE_WRITER_LOG_FLAG log)
• A new single-value accessor cpuWallTiming(CpuWallTimingType)

The underlying cpuWallTimingList_ storage stays protected.

Why now

This is the natural follow-up to #12083 (which made the per-stage timers actually record real numbers) and to #12107 (which adds a per-batch input-encoding counter). Together, the three changes let a reviewer answer "what shape did the writer see, where did the time go, and what inner path drove it" from a single VELOX_SHUFFLE_WRITER_LOG_FLAG run, instead of being stuck at "SplitRV was 65 % of wall time, somewhere in there".

A per-query TPC-DS/TPC-H SF10 profile sweep on a closely-related internal branch shows SplitRV is the top-1 or top-2 bucket on 10/12 queries surveyed (46-71% of wall), with substantially different sub-stage breakdowns: bigint-heavy queries (q17, q14a) are overwhelmingly splitFixedWidthValueBuffer, while VARCHAR-heavy q67 shifts toward splitBinaryArray. Without the sub-stage timers, those breakdowns are invisible.

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: two batches in, outer SplitRV bucket has count == 2. Confirms the new inner timers refine, not replace, the existing outer timer.