fix: address audit findings — fitting trigger, race guards, state conflicts

wear-check:
- Send weak pulse light (brightness=26) to trigger FFE4 fitting report
- Timeout degrades to pass (backward compat with 17-byte firmware)
- Sync _resolved flag prevents double-resolve

treating:
- Only treat run_state=0x00 as completion after seeing run_state=0x02
  (prevents false completion on startup)
- Sync _finishing flag replaces async data.completed for race prevention
- onStatus preserves paused state during fitting loss (|| fittingLost)
- Remove dead _fittingTimer reference from onUnload

auto-scan:
- Sync _completed flag in _scanComplete prevents double-build

miniprogram/pages/auto-scan/auto-scan.js

@@ -43,6 +43,7 @@ Page({
   startScan: function () {
     var self = this
     self._scanResults = []
+    self._completed = false
 
     if (!ble.isConnected()) {
       self.setData({ scanning: false, error: '设备未连接，请先连接设备' })
@@ -110,6 +111,8 @@ Page({
   },
 
   _scanComplete: function () {
+    if (this._completed) return
+    this._completed = true
     this._cleanup()
     this.setData({ scanning: false, scanProgress: 100 })
     this._buildDisplay()