Fix CopyStrokesAction ID tracking and cache grid bitmap

- CopyStrokesAction: use insertAll return value (List<Long>) to get
  inserted IDs directly instead of fragile takeLast(n) re-fetch
- StrokeDao.insertAll now returns List<Long>
- Grid bitmap cached separately from stroke bitmap, redrawn only on
  zoom/pan/resize changes. Eliminates per-frame grid line rendering
  during active drawing.

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

app/src/main/kotlin/net/metacircular/engpad/data/db/StrokeDao.kt

@@ -11,7 +11,7 @@ interface StrokeDao {
     suspend fun insert(stroke: Stroke): Long
 
     @Insert
-    suspend fun insertAll(strokes: List<Stroke>)
+    suspend fun insertAll(strokes: List<Stroke>): List<Long>
 
     @Query("SELECT * FROM strokes WHERE page_id = :pageId ORDER BY stroke_order ASC")
     suspend fun getByPageId(pageId: Long): List<Stroke>

app/src/main/kotlin/net/metacircular/engpad/data/repository/PageRepository.kt

@@ -49,7 +49,7 @@ class PageRepository(
 
     suspend fun deleteStrokes(ids: List<Long>) = strokeDao.deleteByIds(ids)
 
-    suspend fun insertStrokes(strokes: List<Stroke>) = strokeDao.insertAll(strokes)
+    suspend fun insertStrokes(strokes: List<Stroke>): List<Long> = strokeDao.insertAll(strokes)
 
     suspend fun getNextStrokeOrder(pageId: Long): Int =
         (strokeDao.getMaxStrokeOrder(pageId) ?: 0) + 1

app/src/main/kotlin/net/metacircular/engpad/ui/editor/PadCanvasView.kt

@@ -43,6 +43,11 @@ class PadCanvasView(context: Context) : View(context) {
     private var backingCanvas: Canvas? = null
     private var bitmapDirty = true
 
+    // --- Cached grid bitmap (redrawn only on zoom/pan/resize) ---
+    private var gridBitmap: android.graphics.Bitmap? = null
+    private var gridCanvas: Canvas? = null
+    private var gridDirty = true
+
     // --- In-progress stroke ---
     private var currentPath: Path? = null
     private var currentPaint: Paint? = null
@@ -257,6 +262,7 @@ class PadCanvasView(context: Context) : View(context) {
         super.onSizeChanged(w, h, oldw, oldh)
         rebuildViewMatrix()
         bitmapDirty = true
+        gridDirty = true
     }
 
     override fun onDraw(canvas: Canvas) {
@@ -273,8 +279,9 @@ class PadCanvasView(context: Context) : View(context) {
             )
         }
 
-        // Draw grid in screen space with pixel-snapped positions
-        drawGridScreenSpace(canvas)
+        // Draw cached grid
+        ensureGrid()
+        gridBitmap?.let { canvas.drawBitmap(it, 0f, 0f, null) }
 
         // Draw completed strokes from backing bitmap (screen resolution)
         ensureBacking()
@@ -376,6 +383,25 @@ class PadCanvasView(context: Context) : View(context) {
         }
     }
 
+    private fun ensureGrid() {
+        val w = width
+        val h = height
+        if (w <= 0 || h <= 0) return
+
+        if (gridBitmap == null || gridBitmap!!.width != w || gridBitmap!!.height != h) {
+            gridBitmap?.recycle()
+            gridBitmap = androidx.core.graphics.createBitmap(w, h)
+            gridCanvas = Canvas(gridBitmap!!)
+            gridDirty = true
+        }
+
+        if (gridDirty) {
+            gridBitmap!!.eraseColor(android.graphics.Color.TRANSPARENT)
+            drawGridScreenSpace(gridCanvas!!)
+            gridDirty = false
+        }
+    }
+
     private fun drawGridScreenSpace(canvas: Canvas) {
         val pageW = canvasState.pageSize.widthPt.toFloat()
         val pageH = canvasState.pageSize.heightPt.toFloat()
@@ -1024,6 +1050,7 @@ class PadCanvasView(context: Context) : View(context) {
 
         viewMatrix.invert(inverseMatrix)
         bitmapDirty = true
+        gridDirty = true
     }
 
     private fun screenToCanonical(screenX: Float, screenY: Float): FloatArray {

app/src/main/kotlin/net/metacircular/engpad/undo/SelectionActions.kt

@@ -86,12 +86,10 @@ class CopyStrokesAction(
                 createdAt = System.currentTimeMillis(),
             )
         }
-        repository.insertStrokes(copies)
-        // Re-fetch to get assigned IDs
-        val allStrokes = repository.getStrokes(pageId)
-        val newIds = allStrokes.takeLast(copies.size)
-        insertedIds.addAll(newIds.map { it.id })
-        onExecute(newIds)
+        val ids = repository.insertStrokes(copies)
+        insertedIds.addAll(ids)
+        val inserted = copies.zip(ids) { stroke, id -> stroke.copy(id = id) }
+        onExecute(inserted)
     }
 
     override suspend fun undo() {