feat(linalg): make SVD sweep budget configurable - #235

Please drop d8a6370 and 5b7fbaa — that work already landed in #172 (same get_3d / get_3d_custom + helix test). Rebase onto current main so this PR is only the SVD change. Note: on main those APIs were later renamed in #199 (flux_integral_3d / …), so these commits will also conflict/stale.

The SvdSettings / svd_with_settings direction looks right for #156, but svd.rs needs a clean pass before merge:

• There’s an unfinished placeholder left in the docs ([...existing doc comments yahan rakho jaise the...]), and the svd rustdoc got duplicated/broken when the new methods were added.
• Formatting/indentation around svd and svd_with_settings is messed up (lots of blank lines, methods not indented with the rest of the impl). Please tidy / rustfmt.
• Rebase onto current main — svd there already uses self.is_finite(); keep that instead of the old nested loop.
• Please pub use SvdSettings next to Svd, and add a small test that a custom max_sweeps actually changes behaviour.
• Please add a CHANGELOG.md entry under [Unreleased] → ### Added for the configurable SVD sweep budget / SvdSettings.

Happy to re-review once it’s just the SVD change on top of main.

@mubasharameen485-cloud please focus on merging this issue first, do not begin new PRs.