Worker thread calls pauseAndSnapshot with an extractor closure;
the Pauser posts a frame callback that runs extractors on the
main thread and then blocks that thread on a semaphore until
release() fires. FrameCallbackPoster abstracts Choreographer so
JVM unit tests drive it with a single-thread executor, while the
Android runtime uses ChoreographerPoster.
SocketClient owns a background reader thread, dispatches incoming
messages to a handler, and auto-reconnects with exponential backoff
when the transport drops. AgentTransport abstracts the socket so
JVM unit tests drive the client through piped streams while the
Android runtime uses LocalSocket over the abstract namespace.
Send is exposed to consumers via a MessageSender passed to
onConnected. Writes are serialized under the output stream lock.
Uses org.json (already on Android; org.json:json test dep for JVM
unit tests). Field names and framing mirror internal/agent/protocol.go
so Go-encoded frames decode on the SDK without a shared schema file.
pj