add sentence service

android/README.md

@@ -17,6 +17,10 @@ and a placeholder interface for future imitation-quality scoring.
 - Swipe right: next sentence; if there is no sentence data, fast-forward by the
   configured seek step.
 
+The sample app's previous/next sentence buttons use the same fallback behavior.
+Actual sentence navigation requires `SentenceBoundary` timestamps supplied by
+the course service or extracted from subtitle/speech analysis data.
+
 These defaults match the desktop player's arrow-key workflow while fitting a
 tablet touch screen.
 
@@ -62,6 +66,50 @@ OralTrainerSdk.init(
 )
 ```
 
+## Sentence Boundary API
+
+The SDK defaults to `https://video_service.d1kt.cn` and requests:
+
+```text
+GET /api/v1/videos/{sha256}/sentence-boundaries
+```
+
+For a known hash:
+
+```kotlin
+sdk.sentenceBoundaryApi.fetch(videoHash, callback)
+```
+
+For a local `content://` video, the SDK can hash the file in streaming chunks
+before querying the API:
+
+```kotlin
+sdk.sentenceBoundaryApi.fetchForUri(videoUri, contentResolver, callback)
+```
+
+Override the service only when a staging or private deployment is required:
+
+```kotlin
+OralTrainerSdk.init(
+    context,
+    OralTrainerSdkConfig(
+        sentenceBoundaryApiBaseUrl = "https://video_service.d1kt.cn"
+    )
+)
+```
+
+## Local Video Testing
+
+The computer path `/Users/...` is not visible to an Android device. For a
+quick test, copy the video to the device or use the `选择本地视频` button in the
+sample app. The Android document picker returns a `content://` URI, which the
+SDK supports without requesting broad storage permissions.
+
+For production courses, keep the video on an HTTPS CDN or object-storage
+service and pass its URL as `TrainingMediaItem.uri`. The SDK streams it and
+caches downloaded ranges locally. Copying large 4K files to each device is
+better reserved for explicitly offline courses.
+
 ## Future Imitation Scoring
 
 Provide an implementation of `ImitationQualityAssessor` when the speech