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