Monotonic batch id — bumps on every change even if the path set repeats, so a
subscriber re-fires for a second edit to the same file (the value is never
deduplicated away). 0 is the pre-first-event initial.
Repo-relative paths (leading slash, e.g. /src/App.tsx) that just changed.
One working-tree change batch the host pushes: the changed paths plus an epoch.