Added SVD-based projective Wannierization API

[Alphaharrius]

Summary

This PR adds a documented SVD-based projection API for momentum-sector gauge alignment and builds a higher-level proj_wannierization() helper on top of it.

Together, these changes make the projective Wannierization workflow easier to call, better documented, and better tested, while also tightening the lattice-inference path used to rebuild Wannier-style output bases from source offsets.

What changed

• introduced svd_projection() as the explicit API for sectorwise gauge alignment
• exported svd_projection() from qten.ops
• documented the sectorwise SVD / polar-factor construction in detail, including rectangular-rank handling and zero-padded sectors
• clarified and regression-tested the infer_lattice=True path that rebuilds an output lattice from source-side offset labels
• updated Wannier/Bloch matching to use fractional offset tuples rather than ambient-space-dependent offset identity
• added proj_wannierization() as a thin helper that performs:
  • Fourier transform of localized real-space trial orbitals into momentum space
  • projection of target band states into the seed-selected gauge via svd_projection()
• validated tensor ranks and leading-dimension types in proj_wannierization()
• documented the projective Wannierization workflow, tensor-leg conventions, and lattice-aware output behavior
• added regression tests for zero-padded projection behavior
• added tests for rebasing offsets before inferred unit-cell extraction
• added tests for lattice-backed inferred output labels
• added tests showing proj_wannierization() matches explicit Fourier-plus-SVD construction

Why

This branch turns a lower-level projection workflow into a clearer public API surface.

At the lower level, svd_projection() gives the momentum-sector projection logic a stable name, explains the underlying math, and makes the lattice-inference behavior more predictable and reviewable.

At the higher level, proj_wannierization() removes repeated boilerplate for the common workflow of taking localized trial orbitals, transforming them into momentum space, and using them to fix the gauge of a target band manifold.

The fractional-coordinate matching change also makes the inferred Wannier construction less sensitive to symbolic ambient-space identity and better aligned with the intended geometric equivalence.

Testing

• added coverage showing svd_projection() ignores zero-padded filled bands without spurious warnings
• added coverage showing the projection ignores zero-padded target/source columns sectorwise
• added coverage for offset rebasing before inferred unit-cell construction
• added coverage verifying inferred output offsets remain lattice-backed when infer_lattice=True
• added coverage showing proj_wannierization() matches explicit fourier_transform(...) plus svd_projection(...)
• added coverage showing proj_wannierization() can return lattice-aware output when the seed columns carry Hilbert labels

Notes for reviewers

• proj_wannierization() is intentionally a thin orchestration layer; the core numerical alignment remains centralized in svd_projection()
• the main semantic change beyond docs and API naming is in the lattice-inference path, where matching now depends on fractional coordinates rather than raw offset ambient-space equality

[eyjafjallac]

LGTM