Tombstone Native SDK v0.3.0

Event + metric batching: tombstone_track_metric + bounded drop-oldest buffers (cap 256) flushed off-thread on count/age/shutdown to /ingest/events:batch and /ingest/metrics:batch. Each item keeps its own occurredAtIso; the envelope carries sentAtIso. Crashes/bugs/heartbeats remain individual + write-ahead durable. Zero idle allocation; CI green on Windows/Linux/macOS.