fix(executive): reject placeholder unit cells in symexp

[JarrettSJohnson]

Summary

Fixes #509 by preventing symexp from generating symmetry mates for invalid or placeholder unit cells, such as the 1 × 1 × 1 Å P 1 cells in 8GHV and 9HFL. Normal crystallographic expansion is unchanged.

Testing

• Reproduced 26 unwanted mates before the fix and none afterward for 8GHV and 9HFL.
• Verified 1oky still generates the expected symmetry mates.
• Added a regression test for placeholder unit cells.
• All 24 CI checks pass.