perf: optimize flux frac for interpolated images by removing fori_loop

[beckermr]

This PR removes the fori_loop used to compute the flux fraction for interpolated images.

[codspeed-hq]

CodSpeed Performance Report

Merging #134 will degrade performances by 33.9%

_{Comparing opt-flux-frac (3abd2f0) with main (9b2c0b7)}

Summary

⚡ 2 improvements
❌ 1 regressions
✅ 22 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_benchmark_interpimage_flux_frac[compile]	282.5 ms	235.4 ms	+20.02%
❌	test_benchmark_interpimage_flux_frac[run]	1.8 ms	2.7 ms	-33.9%
⚡	test_benchmark_spergel_init[run]	165.2 µs	83.4 µs	+97.94%