Pull-based contract a select-shell host implements so individual
CngxOption instances can render an infrastructure indicator (commit
pending spinner, commit error glyph) inside a reserved internal slot
AFTER the user's option content.
Pull semantics keep the host stateless from the option's perspective:
the option calls host.statusFor(value) once and consumes the returned
Signal<CngxOptionStatus | null> reactively; the host owns the policy
for resolving which value gets which status at any given commit-id.