[lan496]

Summary

Mixed magnetic sites could not be exported faithfully. Both writers now consume
ordinary pymatgen Structure data with complete constituent chemistry,
Cartesian magmom, and an optional boolean magnetic property. Use coincident
rows for partial magnetic selection. SpinCifWriter.from_structure matches
these rows to the SSA model and accepts proper or improper spin frames;
from_oriented preserves the supplied chemistry. Published array and occupancy
interfaces remain supported.

The reader preserves constituent rows, including unequal coincident moments and
explicit magnetic zeros. Writers group equivalent rows for their symmetry
backends and require a shared selected vector. Typed species identities and
lossless decimal output preserve fractional oxidation states, occupancies, weak
moments, and general-origin translations. Caller tolerances control position,
moment, and frame checks. Malformed loops and loss during pymatgen conversion
raise explicit errors.

Test plan

• Full core suite: 269 passed.
• All 122 MCIF/spinCIF tests passed under python -O.
• Independent checks cover fractional-charge collisions, selected-zero
  read/write preservation, equivalent constituent partitions, trace occupancy
  loss, reordered SSA matching, legacy occupancy overrides, nonzero-k
  antitranslations, and proper/improper frames.
• Strict translation reproduction now has zero displacement at
  symprec=1e-11, versus 4e-9 before replacement. Weak 1e-10 components
  survive serialization.
• All pre-commit hooks passed; documented mixed-site example executed.