refactor(spillover): route stage-1 FE through shared _iterative_fe_solve #2752

Job	Run time
Rust Unit Tests (ubuntu-24.04-arm)	1m 31s
Rust Unit Tests (windows-latest)	2m 23s
Rust Unit Tests (ubuntu-latest)	1m 25s
Python Tests (windows-latest, py3.11)	1h 1m 18s
Rust Unit Tests (macos-latest)	1m 16s
Pure Python Fallback	1h 13m 32s
Python Tests (macos-latest, py3.14)	34m 40s
Python Tests (ubuntu-24.04-arm, py3.14)	32m 5s
Python Tests (macos-latest, py3.11)	30m 17s
Python Tests (ubuntu-latest, py3.13)	1h 11m 7s
Python Tests (ubuntu-24.04-arm, py3.11)	33m 12s
Python Tests (windows-latest, py3.14)	1h 3m 3s
Python Tests (ubuntu-latest, py3.11)	59m 8s
Python Tests (macos-latest, py3.13)	34m 44s
Python Tests (ubuntu-latest, py3.14)	1h 9m 27s
Python Tests (windows-latest, py3.13)	55m 12s
Python Tests (ubuntu-24.04-arm, py3.13)	31m 45s
	10h 56m 5s

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