tcp: simplified Policy implementation based on @MDr164 (#190)

* begin prepping policy refactor manually

* implement tcp.Policy and refactor rto to use it

* fix CI

* chatting with claude gave me idea to reformulate Policy

* tcp.Policy: add newTransmitLimit output

* merge with main and fix failing tests

* remove fix.patch

* answer my own comments

tcp/conn.go

@@ -76,16 +76,10 @@ type ConnConfig struct {
 	// Logger sets the [Conn] logger.
 	// Lower level logging available at [Handler.SetLoggers] via [Conn.InternalHandler].
 	Logger *slog.Logger
-	// LossRecovery is the optional packet-loss recovery algorithm (RTO,
-	// congestion control, ...) for the connection. If set, Nanotime must also be
-	// set (else Configure returns an error). Leaving it nil disables loss
-	// recovery. See [LossRecovery].
-	LossRecovery LossRecovery
-	// Nanotime is the monotonic time source in nanoseconds (the func() int64
-	// convention used across lneto) that drives LossRecovery. It is required when
-	// LossRecovery is set and unused otherwise. The tcp package reads it only to
-	// stamp the loss-recovery hooks; it holds no clock itself.
-	Nanotime func() int64
+	// Policy is the optional transmit-steering algorithm (RTO, congestion
+	// control, ...) for the connection. nil disables it. A Policy needing time
+	// carries its own clock. See [Policy].
+	Policy Policy
 }
 
 // Configure should be called on any newly created connection before usage. See [ConnConfig].
@@ -93,10 +87,6 @@ func (conn *Conn) Configure(config ConnConfig) (err error) {
 	if config.RWBackoff == nil {
 		return lneto.ErrMissingHALConfig
 	}
-	if config.LossRecovery != nil && config.Nanotime == nil {
-		// The tcp package holds no clock: a loss-recovery algorithm cannot run without it.
-		return lneto.ErrInvalidConfig
-	}
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
 	err = conn.h.SetBuffers(config.TxBuf, config.RxBuf, config.TxPacketQueueSize)
@@ -105,7 +95,7 @@ func (conn *Conn) Configure(config ConnConfig) (err error) {
 	}
 	conn._backoff = config.RWBackoff
 	conn.logger.log = config.Logger
-	conn.h.SetLossRecovery(config.LossRecovery, config.Nanotime)
+	conn.h.SetPolicy(config.Policy)
 	return nil
 }