v1.2.0

• Add TrackingObserverInterface signals for buffered, written, failed flush,
  and dropped events, with a null default wired through config-plugin.
• Add opt-in schema-v1 retention/TTL templates with explicit enable and disable
  operations; upgrades do not activate data deletion.
• Replace constructor-only benchmarks with append, threshold-flush, and
  large-buffer workloads.
• Route writes through a fan-out-capable internal batch sink and expose an
  opt-in secondary sink without shipping or enabling schema v2.