Long-lived, supervised directory watcher (development workflow).

Modelled as a gen_statem with two states:

• idle: steady state, waiting for filesystem activity.
• debouncing: a relevant change arrived; a state_timeout coalesces a burst of events (editor write-then-rename, formatters, git checkouts) into a single migration run.

One watcher is started per entry in the watchers application environment (see migraterl_sup). It uses the optional fs application to observe the source directories of a run configuration and re-runs migraterl:migrate/2 after the debounce window.

Meant for local development ("apply on save"); production deploys should call migraterl:migrate/2 explicitly at boot.

A watcher spec is a map:

#{conn => epgsql:connection() | epgsql:connect_opts(),
  migrate => map(),            % the migraterl:migrate/2 options map
  debounce_ms => non_neg_integer()}