GH-45847: [C++][Acero] Two-pass merge in GroupByNode to amortise kernel resizes

[Periecle]

The hash-aggregate Merge() step was a single pass that, for every
contributing thread, fed its uniques into state0->grouper and then iterated
every kernel — calling kernel->resize(state0->grouper->num_groups())
once per (thread, kernel) pair. With N threads and K kernels that is
N*K resize calls, while only K are needed (the grouper grows monotonically
so the only resize that does real work is the final one).

This commit implements the two-pass algorithm noted in the existing
in-code TODO at lines 285-289:

Pass 1 Compute every contributing thread's transposition into
state0->grouper. After this pass the grouper is fully grown.
Pass 2 For each kernel, resize state0->agg_states[k] once to the
final group count, then merge every contributing thread's
kernel state in turn.

Semantically equivalent to the previous code: the grouper grows
monotonically across Consume() calls, so each thread's transposition
remains a valid index into state0's final kernel state. Per-row
output (count, sum, etc.) is unchanged.

Verified with a join-then-aggregate reproducer of apacheGH-45847 that
exercises the multi-threaded merge path on a worst-case unique-key
inner join (build x probe = 64 x 64 batches of 32 768 rows = ~2M
groups across 4 worker threads). out_rows and per-group counts match
the unmodified Merge() exactly.

The catastrophic 9000x slowdown reported in apacheGH-45847 itself is largely
addressed by the JoinResultMaterialize::Flush task group that landed
in apache#45918; this change is the complementary aggregator-side fix called
out by the in-code TODO and reduces resize() call count in Merge() from
O(num_threads * num_kernels) to O(num_kernels).

Reproducer:
cpp/src/arrow/acero/repro_45847.cc

Standalone program that wires source(probe) + source(build) ->
hashjoin -> aggregate(hash_count) and times the run. Validates the
output against the analytical expected count so it can be used to
check correctness of further changes.

https://claude.ai/code/session_01BX3n7pizaHw9yVpSetFmXo