docs(filter): document multi-state selection and reconcile the views boundary

I documented the new multi-state capability and brought the views spec and README back in sync with it. The README's filter section notes the State prompt is multi-select and shows the (:state ("Todo" "In Review")) form.

The local/Linear views spec's reverse-compile dimension table, singular-vs-plural rule, and refusal list now list state.name.in as a plural-capable exception alongside :labels and :state-type, and the README copy-down paragraph no longer claims a multi-value filter on any single dimension refuses. The views vNext note records that the named-states case shipped here.

Phase 3 of docs/multi-state-filter-spec.org. Docs only.

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