Holds the merged FormFieldConfig every CngxFormField in scope reads for
its defaults. Each slice comes from one with* feature:
error messages - withErrorMessages
constraint-hint formatters - withConstraintHints
required marker - withRequiredMarker
autocomplete map - withAutocompleteMappings
spellcheck-off set - withNoSpellcheck
error-visibility strategy - withErrorStrategy
Populated by provideFormField(...). The default factory returns an empty
config, so without a provider every feature stays off and the presenter uses
its built-in behaviour (error gate touched OR reveal, no hints, no marker).
Read it through injectFormFieldConfig(). App-wide configuration goes
through provideFormField; a sub-tree override goes through
provideFormFieldAt on a component's providers / viewProviders. Both
merge with* features the same way.