Discussion on kernel execution domain

Assembly of expressions depending on entity data not deducible from single adjacent cell needs a general concept for assembler routines and kernel design.

Examples include
- interior facets in branching meshes,
- vertex integrals, and
- ridge intergals.

Most difficult examples include
- `ufl.jump`,
- `ufl.average`,
- or custom combinations of restrictions.

These involve changes at the level of code generation, in particular data layout is not compile time known.

See also discussion started at https://github.com/FEniCS/dolfinx/pull/3726#issuecomment-3194312836 (and below).