Rewrite Gini coefficient in O(n log n)

[russlan23]

Replaces the quadratic pairwise implementation with the sorted closed-form calculation requested in #926. This removes the module's only prange use and the parallel=True compilation path that blocks the in-browser runtime.

The existing Pareto and Weibull assertions remain unchanged. I added a fixed-array regression against the pairwise definition.

Validation:

• pytest -q quantecon/tests/test_inequality.py — 4 passed
• flake8 --select=F401,F405,E231 quantecon
• Independent agreement checks against the pairwise definition for n = 4, 10, 100, and 1,000

The separate Phase 0 browser deployment in #928 is still open, so that project-level browser check remains outstanding.

Closes #926.

AI assistance: Used for implementation support and test execution; I reviewed the diff and validation results.

[mmcky]

thanks for your PR @russlan23

@kp992 can you please review this as part of the WASM project

[Copilot]

Pull request overview

This PR rewrites gini_coefficient in quantecon/_inequality.py from a quadratic pairwise double-loop into the O(n log n) sorted closed-form calculation. This removes the module's only use of Numba's @njit(parallel=True)/prange, which is the ParallelAccelerator path that fails in the single-threaded in-browser (emscripten) Numba runtime, while also delivering a large algorithmic speedup. It addresses issue #926 as part of the Phase 1 WASM/browser-compliance work.

Changes:

• Replaced the pairwise abs double loop with the sorted formula 2·Σ(i·y_i)/(n·Σy) − (n+1)/n, and dropped the parallel=True flag and prange.
• Narrowed the Numba import to from numba import njit.
• Added a fixed-array regression test pinning the closed form against the pairwise definition.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
quantecon/_inequality.py	Rewrites gini_coefficient to the O(n log n) sorted closed form and removes the prange/parallel=True usage and import.
quantecon/tests/test_inequality.py	Adds a regression test asserting agreement with the pairwise Gini definition on a small fixed array.

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[coveralls]

coverage: 90.608% (+0.04%) from 90.57% — russlan23:agent/rewrite-gini-nlogn into QuantEcon:main