macOS: do not reset the box on connect

On macOS/Linux the OS asserts DTR and RTS when the port is opened and
pyserial then writes DTR before RTS, passing through RTS=1/DTR=0 - the
USB-Serial-JTAG reset state. Every Connect restarted the box. Drop RTS
before open and DTR right after it on posix; Windows is unchanged.

Verified on macOS 27 / arm64 with box A4:CB:8F:C9:47:3C (fw 1.0.16,
build 3ae273ec): connect keeps the box running, Reset & capture still
restarts it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_manager.py

@@ -86,6 +86,46 @@ def test_hard_reset_pushes_rts_via_dtr_write(monkeypatch):
     assert all(c[1] is False for c in ser.calls if c[0] == "dtr")
 
 
+class _PosixOpenSerial:
+    """pyserial on macOS/Linux: the OS asserts both lines on open, then
+    pyserial writes the requested DTR, then RTS. Records every line state."""
+
+    def __init__(self, *args, **kwargs):
+        self.port = args[0] if args else "fake"
+        self.dtr = self.rts = True  # pyserial defaults
+        self.states = []
+
+    def __setattr__(self, name, value):
+        object.__setattr__(self, name, value)
+        if name in ("dtr", "rts") and getattr(self, "is_open", False):
+            self.states.append((self.dtr, self.rts))
+
+    def open(self):
+        want_dtr, want_rts = self.dtr, self.rts
+        object.__setattr__(self, "dtr", True)
+        object.__setattr__(self, "rts", True)
+        self.is_open = True
+        self.states.append((True, True))
+        self.dtr = want_dtr
+        self.rts = want_rts
+
+
+def test_open_on_posix_never_passes_through_reset(monkeypatch):
+    monkeypatch.setattr(mgr_mod.sys, "platform", "darwin")
+    monkeypatch.setattr(mgr_mod, "find_port", lambda sn, dev: LOOP)
+    opened = []
+    monkeypatch.setattr(
+        mgr_mod.serial, "serial_for_url",
+        lambda *a, **k: opened.append(_PosixOpenSerial(*a, **k)) or opened[-1],
+    )
+    m = SerialManager(lambda ln: None, lambda s, d: None)
+    m._target = mgr_mod._Target(LOOP.device, None)
+    ser = m._open()
+    # (DTR, RTS) = (False, True) resets the chip on connect.
+    assert (False, True) not in ser.states
+    assert ser.states[-1] == (False, False)
+
+
 def test_waits_when_port_missing(monkeypatch):
     monkeypatch.setattr(mgr_mod, "find_port", lambda sn, dev: None)
     states, lines = [], []