[PERF] Use prefix sums for conditional_join range aggregations (float dtypes)

[samukweku]

Brief description

Follow-up to #1648, which restricts the prefix-sum rewrite of the forward
range-sum kernels used by join_agg to integer dtypes.

This issue tracks extending the same optimization to float32/float64,
which is deliberately out of scope for #1648.

Relevant Rust implementations:

• src/aggs/sum/sum_starts.rs
• src/aggs/sum/sum_ends.rs
• src/aggs/sum/sum_starts_ends.rs

Relevant Python dispatch:

• janitor/functions/_conditional_join/_agg_functions.py
• janitor/functions/_conditional_join/_get_join_aggs.py

Why this is separate from #1648

Some current Rust kernels use compensated summation (e.g. Kahan/Neumaier) for
floats. A naive prefix-sum-and-subtract rewrite can change rounding or
cancellation behavior versus the current output, so this isn't a drop-in
replacement the way it is for integers.

Proposed scope

1. Define and test an explicit numerical-accuracy contract for float prefix
   sums (e.g. acceptable ULP/relative-error bounds vs. the current
   compensated-summation kernels), or decide compensated summation must be
   preserved and design a prefix-sum variant that keeps it (e.g. a
   compensated prefix sum with a compensated subtraction step).
2. Implement the NumPy (or Rust) prefix-sum helper for float32/float64
   covering suffix, prefix, and arbitrary [start:end) ranges, mirroring the
   integer implementation landed in [PERF] Use prefix sums for conditional_join range aggregations (integer dtypes) #1648.
3. Add focused unit tests for rounding/cancellation parity, null handling,
   empty ranges, and extension-array reconstruction.
4. Add benchmarks comparable to the ones gathered for [PERF] Use prefix sums for conditional_join range aggregations (integer dtypes) #1648.

Acceptance criteria

• Float results match the current implementation within the agreed
  numerical-accuracy contract, across null placements, empty inputs, and
  range shapes.
• The full test suite passes.
• Benchmarks demonstrate an end-to-end improvement, not only a kernel-level
  improvement.
• The changelog entry references this issue number, the implementing PR
  number, and @samukweku.

Related aggregation work:

• [PERF] Use prefix sums for conditional_join range aggregations (integer dtypes) #1648 (integer prefix sums for sum, this issue's prerequisite/sibling)
• [PERF] Optimize conditional_join range min/max aggregations #1653 reviews forward range min/max algorithms.
• [PERF] Review conditional_join range product aggregations #1654 reviews forward range prod algorithms and numerical contracts.
• [PERF] Make reverse aggregations deterministic and dense janitor-rs#23 covers deterministic retained reverse
  aggregation kernels.