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>

kota_tech/serialio/manager.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 
 import enum
 import queue
+import sys
 import threading
 import time
 from collections.abc import Callable
@@ -92,13 +93,21 @@ class SerialManager:
         ser = serial.serial_for_url(
             info.device, baudrate=self._baud, timeout=READ_TIMEOUT_S, do_not_open=True
         )
-        # Deassert both lines before open. On USB-Serial-JTAG, RTS=1/DTR=0
-        # holds the chip in reset and DTR=1 during reset selects download
-        # mode; pyserial would otherwise assert both on open (Windows).
-        ser.dtr = False
+        # On USB-Serial-JTAG, RTS=1/DTR=0 holds the chip in reset and DTR=1
+        # during reset selects download mode.
+        # Windows: pyserial applies both lines atomically at open, so
+        # deassert both beforehand (it would otherwise assert both).
+        # macOS/Linux: the OS asserts both lines on open and pyserial then
+        # writes DTR before RTS, passing through RTS=1/DTR=0 and resetting
+        # the box on every connect. Drop RTS first, then DTR.
+        posix = sys.platform != "win32"
+        if not posix:
+            ser.dtr = False
         ser.rts = False
         try:
             ser.open()
+            if posix:
+                ser.dtr = False
         except (serial.SerialException, OSError):
             return None
         self._target.device = info.device  # COM number / node may change