Reusable fields and explicit operators

omnibias-fields supplies reusable field state, views, and backend-specific differential operators. It is a primitive package in the retained workspace; consumer PDE solvers live separately.

Use the field API to identify the representation a function expects. Field-state operators, neural-network layer tuples, and mixed Taylor jets are different input contracts.

A field gradient, divergence, curl, or Laplacian must be interpreted with its scalar/vector shape conventions and coordinate ordering. For a network Laplacian, the direct contraction tutorial provides a small independent check.