Introduce validated AtomData container

[bjmorgan]

Summary

atom_data on StructureScene is a plain dict[str, np.ndarray]. Users can bypass set_atom_data() and write directly to the dict with no validation -- wrong-length arrays, non-arrays, and mixed None/numeric object arrays silently enter the system and cause confusing errors downstream during colour resolution.

Proposed changes

1. AtomData container -- a MutableMapping[str, np.ndarray] that validates on assignment: checks 1-D shape, correct length, and rejects object-dtype arrays containing mixed None + numeric values.

2. Convert StructureScene from dataclass to hand-rolled class -- the current __setattr__ and __post_init__ overrides fight the dataclass machinery. An explicit __init__ with property setters for view, lattice, and atom_data would be cleaner.

Future consideration

Per-frame atom data (shape (n_frames, n_atoms) in addition to the current (n_atoms,)) for trajectory colouring (e.g. per-frame velocities, charges). The AtomData container should be designed so this can be added without breaking changes.