fix(xnet): allocate ephemeral ports sequentially, not randomly (#180)

Ephemeral ports were drawn at random from the 16384-port dynamic range on
every dial. Under connection-per-request churn the birthday paradox reuses
a recently-released port after only a few dozen dials (~1% per dial at 200
outstanding-in-teardown), and a reused 4-tuple lands on whatever state the
previous conversation left along the path, such as the peer's TIME-WAIT
socket or a NAT's flow-table entry, which silently swallows the new SYN.
Measured end to end: an HTTP client with keep-alives off against a macOS
peer hit a dead dial after ~286 connections and stayed dark for ~30
seconds, exactly one 2MSL TIME-WAIT expiry.

Allocate sequentially from a per-stack random starting offset instead: a
port is only revisited after the full 16384-port cycle, and the random
start keeps a rebooted node off the ports its previous life just used.

TestEphemeralPortSequence pins the full-cycle-no-reuse property.

Co-authored-by: Derek den Haas <d.haas@directcode.com>

x/xnet/stack-go.go

@@ -102,8 +102,9 @@ func (s StackGo) SocketNetip(ctx context.Context, network string, family, sotype
 	isDial := raddr.IsValid() && !raddr.Addr().IsUnspecified()
 	if laddr.Port() == 0 {
 		// Auto-assign an ephemeral port for both outbound dials and for listeners
-		// that did not request a fixed port.
-		laddr = netip.AddrPortFrom(laddr.Addr(), uint16(49152+s.blk.async.Prand32()%16384))
+		// that did not request a fixed port. Sequential, not random: see
+		// [StackAsync.ephemeralPort] for why random selection breaks dial churn.
+		laddr = netip.AddrPortFrom(laddr.Addr(), s.blk.async.ephemeralPort())
 	}
 	if laddr.Addr().IsUnspecified() {
 		// Fill in the stack's configured address for the requested family.