Empty state pattern

Purpose-built UI for "no data yet" surfaces — illustration + heading + body + primary action. Treats absence-of-data as a first-class UX moment, not a fallback. Modeled after Polaris (Shopify) EmptyState and Evergreen (Segment) empty-state catalogue.

Pattern — Empty state

*tatus* v0.1.0 — Draft. First entry of the new specs/patterns/ group (resolver in tools/design-gen/internal/spec/parse.go updated to route `patterns