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/policy.go

@@ -0,0 +1,30 @@
+package tcp
+
+// TransmitUnlimited size returned by [Policy.PreTx] to signal no new data transmit limit (no congestion control).
+const TransmitUnlimited = ^Size(0)
+
+// Policy observes segment traffic and steers transmit behaviour: RTO,
+// congestion control and the like (discussion #157). The tcp package holds no
+// clock, so a Policy needing time carries its own (issue #140).
+type Policy interface {
+	// Reset returns the Policy to its pre-connection state. Should be called on every
+	// Open/Listen on connection creation. Configuration like clock setting and fine tuning
+	// should persist throughout the Policy lifetime after Reset calls.
+	Reset()
+
+	// PreTx is called before writing to a frame.
+	// The outgoing frame options can be set by the Policy and will be respected if Frame offset >5.
+	// retransmitFrom is ignored unless within [snd.UNA, snd.NXT] and returned retransmit==true.
+	// newTransmitLimit sets the maximum number of new bytes to send over the wire (congestion control).
+	// If not implementing congestion control then newTransmitLimit=[TransmitUnlimited].
+	PreTx(h *Handler, outgoingOpts Frame) (newTransmitLimit Size, retransmitFrom Value, retransmit bool)
+	// PostTx called on leaving the transmit path with the fully written frame.
+	PostTx(h *Handler, outgoing Frame)
+
+	// PreRx is called by [Handler] on every incoming segment.
+	// PreRx can choose to drop segment if it returns keep=false.
+	PreRx(h *Handler, incoming Frame) (keep bool)
+	// PostRx is called by [Handler] after accepting an incoming segment.
+	// To access [ControlBlock.SendUNA] before incoming frame was processed save UNA in PreRx.
+	PostRx(h *Handler, prevState State, accepted Frame)
+}