refactor(worker): collapse session goroutines into single runSession loop

Replace the five-goroutine session model (RunDialer, RunKeepalive,
RunReader, RunHeartbeatForwarder, RunStopMonitor, Session) with a single
DefaultWorker.runSession method containing two select loops: one
pre-connection and one connected. Ephemeral dial goroutines replace
RunDialer; the keepalive timer and heartbeat reset are inlined. No
exported building-block symbols remain.

Consolidate worker_dialer_test.go, worker_session_test.go, and
worker_start_test.go into worker_test.go. Add seven new behavioral
tests covering dial failure, keepalive-driven dial replacement,
pre-connection stop, message delivery with timestamp, sustained
activity and pong resetting the keepalive timer, keepalive-triggered
reconnect, and nil connection pointer after disconnect.
Update EXTEND.md and README.md to remove references to the deleted

building blocks and document the single worker replacement pattern

README.md

@@ -266,9 +266,9 @@ connStats := conn.Stats() // conn is a *transport.Connection
 
 ## Extending Pools
 
-The pool owns peer registration, event plumbing, and lifecycle. The worker owns what happens on the wire. The default worker can be replaced entirely or composed from the exported `Run*` building blocks that Honeybee provides.
+The pool owns peer registration, event plumbing, and lifecycle. The worker owns what happens on the wire. The default worker can be replaced entirely via `WorkerFactory`.
 
-See EXTEND.md for the worker interface contract, the `PoolPlugin` fields, and the available building blocks for the pool worker.
+See EXTEND.md for the worker interface contract, the `PoolPlugin` fields, and extension patterns.
 
 ## Configuration