feat(gml670-v2): 添加传感器采集线程与主入口

- sensor: 独立 Zephyr 线程，4 通道×17 次均值采集→CoP→BLE 发送，
  原子去皮请求，5Hz 调试日志
- main: 薄编排层，仅初始化 BLE 和传感器模块

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.c

@@ -0,0 +1,28 @@
+/*
+ * GML670 Balance Board — CoP Binary Protocol
+ * Features:
+ * - 4-channel ADS1256 with averaging filter.
+ * - Board-side CoP (Center of Pressure) computation.
+ * - Binary protocol over BLE NUS (17-byte CoP frame, 50 Hz).
+ * - Downlink: resistance params, spotter mode, heartbeat.
+ */
+
+#include "ble_transport.h"
+#include "sensor.h"
+
+#include <zephyr/kernel.h>
+#include <zephyr/logging/log.h>
+
+LOG_MODULE_REGISTER(main, LOG_LEVEL_INF);
+
+int main(void) {
+    k_msleep(2000);
+    LOG_INF("--- GML670 System (CoP Binary Protocol) ---");
+
+    ble_transport_init();
+    ble_transport_adv_start();
+
+    if (sensor_init()) return 0;
+
+    return 0;
+}