Release 1.19.0

The update matrix, complete (roadmap WS-P task 8). release-update.js sends ov:content-dispose (bubbling, { component, release }) on a region just before it is replaced, so a stateless widget inside can drop its listeners and timers; ov:content-updated follows as before, to mount it again. Tests now cover every row of the matrix: stylesheet swap and ov:styles-updated, revision-aware content with scroll kept, widget disposal and remount, deferral of busy regions, the prompt, and the security minimum. The README "Releases" section lists the rows.