Per-key overrides for FormView.fields. overrides draw through the
whole-form view; styles style one key's base field view there. Unknown
keys in either are type errors. A custom layout passes its override to
field itself, with its own h.
readonly attrs?:|{ readonly [K in Key]?: FieldAttrs}|undefined
What one key's element takes beyond the base field view. The text,
multiline, and number renderers read them for a base field view, and a
per-key override is handed them as its own attrs too, so it decides what
to do with them; a bundle-backed kind drawn by bundleRenderer reads none.
Passing field draws through the base view directly, skipping both
renderers and attrs.