fix(rotator): stop rebooting the controller between commands

A K3NG controller answered PuTTY perfectly and told OpsLog 'no reply to
C'. The reason is not the protocol: the client opened and CLOSED the
serial port for every single command, and an Arduino-based controller
resets when its port is opened — DTR pulses the reset pin. OpsLog was
rebooting it several times a second, and every command it sent landed in
a bootloader.

The port is opened once and held, per COM port, at package level: the
callers build a fresh Client per poll, so the port has to outlive them,
and a serial port is a single-owner resource in any case. A newly opened
port is given two seconds to boot before the first command, bytes left
from a previous exchange are drained rather than read as this command's
answer, and a failed exchange drops the port so the next starts from a
clean open instead of repeating the same silence.

Reply parsing was already right for both flavours and now has the real
strings to prove it, that controller's '+0140' among them.

internal/rotator/gs232/reply_test.go

@@ -0,0 +1,34 @@
+package gs232
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// Real replies, as the controllers actually send them — a K3NG answering "C"
+// with "+0140" and CR+LF among them (reported from a live controller).
+func TestAzimuthReplies(t *testing.T) {
+	cases := []struct {
+		raw  string
+		want int
+	}{
+		{"+0140\r\n", 140},         // GS-232A, K3NG firmware
+		{"+0000\r", 0},             // due north
+		{"+0359\r\n", 359},         // just short of it
+		{"AZ=140\r\n", 140},        // GS-232B flavour
+		{"AZ=140 EL=000\r\n", 140}, // GS-232B with elevation on the same line
+		{"\r\n+0075\r\n", 75},      // a leftover terminator ahead of the answer
+	}
+	for _, c := range cases {
+		m := azRe.FindStringSubmatch(strings.TrimSpace(c.raw))
+		if m == nil {
+			t.Errorf("no azimuth found in %q", c.raw)
+			continue
+		}
+		got, _ := strconv.Atoi(m[1])
+		if got%360 != c.want {
+			t.Errorf("%q parsed as %d, want %d", c.raw, got%360, c.want)
+		}
+	}
+}