Write seam between the shared row controller and the component that
owns the expression storage. CngxFilterRow implements it against its
local model(); CngxFilterExpressionRow delegates to the
CNGX_FILTER_BUILDER_HOST path mutators. The controller never writes
state itself - every mutation funnels through exactly one sink call.
Apply a computed field change. When plan.resetValue is false the
carried operator is still valid: only the field changes and operator
and value are kept. When true, the sink must also write
plan.operator and clear the value.