Expose per-ring `dropped_count` / `lost_count` atomics in SubRingHeader
(placed inside existing write_pos cache-line padding — no sizeof change,
no new coherency traffic). Publisher bumps dropped_count on the self-
repair / drop path; Subscriber bumps lost_count at each loss site.

New SharedRegion::stats() returns a RegionStats snapshot: per-ring state,
in_flight, write_pos, dropped_count, lost_count plus aggregates
(total_writes, total_drops, total_losses, live_rings, approximate
pool_free). Safe under live traffic.

Python bindings expose RingStats, RegionStats, and SharedRegion.stats()
so external monitoring can read counters without a CLI.