DONT-MERGE GSOC26: Improve performance of replace_with_mask()

[s7g4]

Change Description

Closes #52

Optimized NestedExtensionArray.__setitem__ and the internal replace_with_mask utility to handle pyarrow.Scalar values directly. This avoids the significant overhead caused by expanding a single replacement scalar into a full-sized pyarrow.Array (matching the length of the target array).

Solution Description

The optimization involves two main changes in src/nested_pandas/series/ext_array.py:

1. NestedExtensionArray.__setitem__: Now detects if the replacement value can be boxed as a pa.Scalar. If so, it passes the scalar directly instead of creating a len(self)-sized array.
2. replace_with_mask: Updated to handle pa.Scalar inputs. When a scalar is detected, it leverages PyArrow's native pa.compute.if_else kernel for efficient broadcasting.

Benchmarks (1,000,000 rows):

• Original approach: ~7.3s execution time | ~78MB peak memory
• Optimized approach: ~0.26s execution time | ~0MB peak memory (approx. 30x faster)

Code Quality

• I have read the Contribution Guide and agree to the Code of Conduct
• My code follows the code style of this project
• My code builds (or compiles) cleanly without any errors or warnings
• My code contains relevant comments and necessary documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.04%. Comparing base (509562f) to head (1dc5fdf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #476   +/-   ##
=======================================
  Coverage   96.03%   96.04%           
=======================================
  Files          20       20           
  Lines        2247     2249    +2     
=======================================
+ Hits         2158     2160    +2     
  Misses         89       89           

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[github-actions]

Before [509562f]	After [29420ea]	Ratio	Benchmark (Parameter)
180M	182M	1.01	benchmarks.ReadFewColumnsHTTPS.peakmem_run
139M	139M	1	benchmarks.CountNestedBy.peakmem_run
106M	105M	1	benchmarks.NestedFrameAddNested.peakmem_run
111M	110M	1	benchmarks.NestedFrameQuery.peakmem_run
11.3±0.3ms	11.3±0.2ms	1	benchmarks.NestedFrameQuery.time_run
109M	109M	1	benchmarks.NestedFrameReduce.peakmem_run
680±50ms	684±10ms	1	benchmarks.ReadFewColumnsS3.time_run
262M	262M	1	benchmarks.ReassignHalfOfNestedSeries.peakmem_run
66.8±2ms	66.1±1ms	0.99	benchmarks.CountNestedBy.time_run
11.5±0.3ms	11.5±0.2ms	0.99	benchmarks.NestedFrameAddNested.time_run

Click here to view all benchmarks.

[delucchi-cmu]

Please include unit tests for the new functionality. Existing test_replace_with_mask only provides list test data.

[delucchi-cmu]

Pre-commit fails for line too long.