android+docs: debug 日志默认关闭；操作手册升级为插图版

debug 日志：DebugLog 增加总开关 enabled（默认 false），log() 直接返回、
init() 不装崩溃钩子、startFile() 不建文件。要在设备上排查时改成 true 重新构建
即可，其它代码不动。关闭后崩溃仍按安卓常规方式可见（logcat AndroidRuntime +
系统弹窗），只是不再有落在相册目录旁的 debug_*.log。

真机烟雾测试（小米 22041211AC，相机插着）：出流正常、拍照成功（相册 19→20，
新增 MAG160C_20260912_153522.jpg）、崩溃 0、App 自身日志行数 0、无新日志文件。

插图版手册 docs/android_app/app_manual.md + manual_images/：
- 02_markers：标记结构解剖（准星/读数/引线/描边字），真机照片放大 3 倍并标注
- 03/04：横屏 960x720 与竖屏 720x960 的真实照片
- 05/06/07：设置、分析（含底部测量面板）、相册 真机截图
- 01_live_schematic：实时界面改为**自绘示意图**，因为实时 OSD 文字按握持角度预旋转
  （用户要求的可读性设计），手册拍摄时手机横放，截图里文字全是侧躺的无法阅读；
  其余界面文字本就水平，直接用截图。

图由 manual_images/MakeFigures.java 生成（裁剪缩放/放大标注/示意图绘制），
界面改动后重跑即可；原始截图存为 JPEG（PNG→JPEG 省约 8MB）作为可再生输入。
.class 已加入 .gitignore。

README 阅读顺序加入操作手册、结构表更新、里程碑补记根因级修复。105 项测试全绿。

android/app/src/main/kotlin/com/mag160c/thermal/MainActivity.kt

@@ -12,8 +12,10 @@ import com.mag160c.thermal.ui.theme.Mag160cTheme
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
-        // field-debug crash hook + log file (opened at startup so even
-        // early crashes are captured; logging never throws)
+        // Field-debug logging is OFF by default (see DebugLog.enabled): a shipping
+        // build writes no log file and no logcat lines. Flip that flag and rebuild to
+        // capture a detailed trace on a device — these two calls then behave exactly
+        // as they used to.
         com.mag160c.thermal.media.DebugLog.init(applicationContext)
         com.mag160c.thermal.media.DebugLog.startFile(applicationContext)
         // enableEdgeToEdge exists since API 21 but reaches for the modern inset

android/app/src/main/kotlin/com/mag160c/thermal/media/DebugLog.kt

@@ -16,8 +16,20 @@ import java.util.Locale
 
 /**
  * Field-debug logger: mirrors every line to logcat AND appends it to a
- * timestamped text file next to the photos (DCIM/MAG160C), so the user can
- * hand over the file without adb (real-device bring-up, round 11).
+ * timestamped text file next to the photos (DCIM/MAG160C), so a problem on the
+ * user's phone can be diagnosed without adb.
+ *
+ * ## It is OFF by default (user decision, 2026-09-12)
+ *
+ * A shipping build has no business writing a log file beside the user's photos or
+ * filling logcat at 15 frames per second, so [enabled] defaults to false and every
+ * entry point returns immediately. To investigate something on a device, flip
+ * [enabled] to true and rebuild — that is the whole procedure; the file then
+ * appears exactly as it used to.
+ *
+ * With logging off, crashes are still reported the normal Android way (logcat
+ * `AndroidRuntime` plus the system's "app keeps stopping" dialog); only the
+ * convenient on-device copy is gone.
  *
  * Logging must NEVER crash the app: every sink operation is guarded, and
  * writes are capped.  Sink fallback chain (scoped storage only allows media
@@ -28,6 +40,13 @@ import java.util.Locale
  * The header line records where the file actually ended up.
  */
 object DebugLog {
+    /**
+     * Master switch. OFF in shipping builds: writes nothing, to no sink.
+     * Set to true (and rebuild) to capture a detailed trace on a device.
+     */
+    @Volatile
+    var enabled: Boolean = false
+
     private val fmt = SimpleDateFormat("HH:mm:ss.SSS", Locale.ENGLISH)
     private val fileFmt = SimpleDateFormat("yyyyMMdd_HHmmss", Locale.ENGLISH)
     private val lock = Any()
@@ -38,6 +57,7 @@ object DebugLog {
 
     /** Install once from MainActivity.onCreate: crash hook + app context. */
     fun init(context: Context) {
+        if (!enabled) return
         synchronized(lock) {
             if (previousHandler != null) return
             previousHandler = Thread.getDefaultUncaughtExceptionHandler()
@@ -51,6 +71,7 @@ object DebugLog {
 
     /** Log one line (also to logcat as MAG160C/<tag>). Never throws. */
     fun log(tag: String, msg: String) {
+        if (!enabled) return
         try {
             Log.d("MAG160C/$tag", msg)
         } catch (_: Exception) {
@@ -80,9 +101,10 @@ object DebugLog {
     /**
      * Open a fresh debug_*.log.  Called once at app start (so even startup
      * crashes are captured); never throws.  Returns the display path for
-     * logging.  Closes any previous file.
+     * logging.  Closes any previous file.  Does nothing when [enabled] is false.
      */
     fun startFile(context: Context): String {
+        if (!enabled) return "(debug log disabled)"
         val name = "debug_${fileFmt.format(Date())}.log"
         closeFile()
         synchronized(lock) {