feat: add stop gradient calls for certain cases for beta/nu in Moffat and Spergel plus docs

[beckermr]

This is more for completeness than anything else.

[codspeed-hq]

Merging this PR will improve performance by 31.79%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 1 improved benchmark
✅ 35 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	test_benchmarks_lanczos_interp[xval-conserve_dc-run]	1,150.3 µs	872.8 µs	+31.79%

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_{Comparing stop-grad-kv-nu (bd23016) with main (8842c01)}