CngxFormFieldHostContract

Narrow back-channel a form-field-aware host (typically CngxFormFieldPresenter) exposes to the bound control.

Two required pieces plus one optional naming channel:

• showError: Signal<boolean> - the resolved "should errors be visible right now" flag. Combines invalid() with the configured visibility gate (touched, dirty, submitted, custom strategy, ambient reveal-scope). The control reads this to decide whether to paint its own error skin (aria-invalid, .cngx-...--error).
• markAsTouched(): void - invoked by the control on focus-out so the surrounding field's "touched" state advances without the control importing the concrete presenter or field-state shape.
• labelId?: Signal<string> - stable id of the field's label element. Group-role atoms (radiogroup / group / listbox / toolbar) reference it via aria-labelledby when no explicit label input is set; group roles take no name from content, so this channel is what names them inside a cngx-form-field.
• skin?: Signal<CngxFieldSkin | undefined> - the field's own, unresolved appearance value. The host publishes what was bound to it and nothing else; resolving the cascade is the reader's job.

Deliberately scoped to these members. The full presenter exposes many more signals (constraints, ARIA IDs, dirty / pending / readonly / submitting); pulling all of them through a token would over-couple the control surface to the presenter's evolution. Anything richer lives inside the form-field bridge directives in @cngx/forms/field, not inside the control atoms themselves.

Layer note. Lives in @cngx/core/tokens (Level 1) so Level-2 atoms in @cngx/common/* can read the contract without importing from the Level-3 @cngx/forms/field library. Mirrors CNGX_FORM_FIELD_CONTROL (atom-side contract) on the host side; together the two tokens form a Sheriff-clean parent-child interaction surface.