perf: CalendarPager alpha() → graphicsLayer 避免滑动重组

月份切换滑动时 pageOffset 每帧变化，alpha() Modifier 触发重组。
改用 graphicsLayer 在 draw 阶段更新透明度，跳过 Composition/Layout。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/src/main/kotlin/plus/rua/project/ui/CalendarPager.kt

@@ -11,7 +11,7 @@ import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.draw.alpha
+import androidx.compose.ui.graphics.graphicsLayer
 import kotlinx.coroutines.flow.drop
 import kotlinx.coroutines.launch
 import kotlinx.datetime.LocalDate
@@ -117,7 +117,7 @@ fun CalendarPager(
             shiftKindAt = shiftKindAt,
             showLegalHoliday = showLegalHoliday,
             onRowHeightMeasured = onRowHeightMeasured,
-            modifier = Modifier.alpha(alpha)
+            modifier = Modifier.graphicsLayer { this.alpha = alpha }
         )
     }
 }