fix(date-recorder): split handwriting strokes on finger lift

endStroke() was a no-op, so addStrokePoint() kept appending every new
pointer-down to the previous stroke (its only segmentation cue was
points.isEmpty()). A second drag after lift got joined into the first,
and toPath() drew them as a single polyline.

Add isFinished flag to HandStroke and implement real endStroke() that
marks the current stroke finished. withAddedPoint() now starts a new
segment when the previous stroke is null, empty, or finished.

core/src/main/kotlin/plus/rua/project/PhotoEditorState.kt

@@ -14,11 +14,14 @@ import androidx.compose.ui.graphics.drawscope.Stroke
  * @param color 笔触颜色
  * @param widthPx 笔触线宽（像素）
  * @param points 采样点序列（基于图片显示坐标系）
+ * @param isFinished 该笔触是否已结束（手指已抬起）。下一条笔触必须另起新段，
+ * 不能再追加到 [points] 上；否则两次手写会被 [toPath] 用一条折线连在一起。
  */
 data class HandStroke(
     val color: Color,
     val widthPx: Float,
-    val points: List<Offset> = emptyList()
+    val points: List<Offset> = emptyList(),
+    val isFinished: Boolean = false
 )
 
 /**
@@ -80,3 +83,30 @@ fun HandStroke.toPath(): Path {
     }
     return path
 }
+
+/**
+ * 在当前状态下追加一个手写采样点，返回新状态。
+ *
+ * 若上一条笔触不存在、为空、或已结束（[HandStroke.isFinished]），则另起新段；
+ * 否则把点追加到上一条笔触的末尾。
+ */
+fun PhotoEditorState.withAddedPoint(offset: Offset): PhotoEditorState {
+    val last = strokes.lastOrNull()
+    val newStrokes = if (last == null || last.points.isEmpty() || last.isFinished) {
+        strokes + HandStroke(strokeColor, strokeWidthPx, listOf(offset))
+    } else {
+        strokes.dropLast(1) + last.copy(points = last.points + offset)
+    }
+    return copy(strokes = newStrokes)
+}
+
+/**
+ * 结束当前正在绘制的笔触，返回新状态。若没有进行中的笔触则原样返回。
+ */
+fun PhotoEditorState.withEndedStroke(): PhotoEditorState {
+    val last = strokes.lastOrNull() ?: return this
+    if (!last.isFinished && last.points.isNotEmpty()) {
+        return copy(strokes = strokes.dropLast(1) + last.copy(isFinished = true))
+    }
+    return this
+}

core/src/main/kotlin/plus/rua/project/PhotoEditorViewModel.kt

@@ -105,22 +105,12 @@ class PhotoEditorViewModel(
 
     /** 追加一个手写笔触采样点。 */
     fun addStrokePoint(offset: Offset) {
-        update {
-            val current = it.strokeColor to it.strokeWidthPx
-            val newStrokes = if (it.strokes.isEmpty() || it.strokes.last().points.isEmpty()) {
-                it.strokes + HandStroke(current.first, current.second, listOf(offset))
-            } else {
-                val last = it.strokes.last()
-                it.strokes.dropLast(1) + last.copy(points = last.points + offset)
-            }
-            it.copy(strokes = newStrokes)
-        }
+        update { it.withAddedPoint(offset) }
     }
 
     /** 结束当前笔触（抬起手指）。 */
     fun endStroke() {
-        // 当前实现笔触在 addStrokePoint 中即时累积，无需额外处理；
-        // 保留方法以便未来区分笔触段（如双击结束）。
+        update { it.withEndedStroke() }
     }
 
     /** 撤销最近一条笔触。 */

core/src/test/kotlin/plus/rua/project/HandStrokeTest.kt

@@ -0,0 +1,109 @@
+package plus.rua.project
+
+import androidx.compose.ui.geometry.Offset
+import sun.misc.Unsafe
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+/**
+ * 手写笔触分段逻辑单元测试。
+ *
+ * 覆盖 [PhotoEditorState.withAddedPoint] / [PhotoEditorState.withEndedStroke]：
+ * 抬手后再次落笔必须另起新段，不能与上一条笔触连成一条折线。
+ */
+class HandStrokeTest {
+
+    private fun emptyState() = PhotoEditorState(
+        sourceBitmap = UninitializedBitmap,
+        sourceAbsolutePath = "/tmp/fake.jpg"
+    )
+
+    @Test
+    fun twoStrokes_afterLiftAreSeparate() {
+        // 画第一笔 A→B
+        var s = emptyState()
+            .withAddedPoint(Offset(0f, 0f))
+            .withAddedPoint(Offset(10f, 0f))
+        // 抬手
+        s = s.withEndedStroke()
+        // 在别处落笔 C→D
+        s = s.withAddedPoint(Offset(100f, 100f))
+            .withAddedPoint(Offset(110f, 100f))
+
+        // 期望：两条独立笔触，而不是 [A,B,C,D] 一条
+        assertEquals(2, s.strokes.size, "两笔应分隔为两段")
+        assertEquals(listOf(Offset(0f, 0f), Offset(10f, 0f)), s.strokes[0].points)
+        assertEquals(listOf(Offset(100f, 100f), Offset(110f, 100f)), s.strokes[1].points)
+    }
+
+    @Test
+    fun points_accumulateWithinSameStroke() {
+        val s = emptyState()
+            .withAddedPoint(Offset(1f, 1f))
+            .withAddedPoint(Offset(2f, 2f))
+            .withAddedPoint(Offset(3f, 3f))
+
+        assertEquals(1, s.strokes.size)
+        assertEquals(
+            listOf(Offset(1f, 1f), Offset(2f, 2f), Offset(3f, 3f)),
+            s.strokes[0].points
+        )
+    }
+
+    @Test
+    fun endStroke_marksCurrentStrokeFinished() {
+        val s = emptyState()
+            .withAddedPoint(Offset(0f, 0f))
+            .withEndedStroke()
+
+        assertEquals(1, s.strokes.size)
+        assertTrue(s.strokes[0].isFinished, "抬手后笔触应标记为已结束")
+    }
+
+    @Test
+    fun endStroke_withNoStrokes_isNoop() {
+        val s = emptyState().withEndedStroke()
+        assertTrue(s.strokes.isEmpty())
+    }
+
+    @Test
+    fun endStroke_calledTwice_keepsStrokeFinished() {
+        val s = emptyState()
+            .withAddedPoint(Offset(0f, 0f))
+            .withEndedStroke()
+            .withEndedStroke()
+
+        assertEquals(1, s.strokes.size)
+        assertTrue(s.strokes[0].isFinished)
+    }
+
+    @Test
+    fun addPoint_afterFinished_startsNewStroke() {
+        // 抬手之后再落点：上一条 isFinished=true，应另起新段
+        val s = emptyState()
+            .withAddedPoint(Offset(0f, 0f))
+            .withEndedStroke()
+            .withAddedPoint(Offset(50f, 50f))
+
+        assertEquals(2, s.strokes.size)
+        assertFalse(s.strokes[0].points.contains(Offset(50f, 50f)))
+    }
+
+    private companion object {
+        /**
+         * 通过 Unsafe.allocateInstance 创建 Bitmap 桩，跳过 Android 框架的静态初始化。
+         * 笔触分段逻辑不读 Bitmap 任何字段/方法，桩仅用于满足 PhotoEditorState 构造器。
+         */
+        @Suppress("DiscouragedPrivateApi", "DEPRECATION")
+        val UninitializedBitmap: android.graphics.Bitmap by lazy {
+            val unsafeField = Unsafe::class.java.getDeclaredField("theUnsafe").apply { isAccessible = true }
+            val unsafe = unsafeField.get(null) as Unsafe
+            @Suppress("UNCHECKED_CAST")
+            val bmp = unsafe.allocateInstance(android.graphics.Bitmap::class.java)
+                as android.graphics.Bitmap
+            bmp
+        }
+    }
+}