refactor(studio): single-source timeline id-resolution and resize-clamp math

Extract resolveTimelineIdForSelection so DOM-to-timeline id mapping lives in one place
with a single sourceFile / activeCompPath / index.html fallback, fixing a sub-composition
selection that previously diverged between callers.

Extract shared start-trim delta helpers used by both single-clip and group resize, and
remove the never-called refreshDomEditGroupSelectionsFromPreview.

packages/studio/src/hooks/useTimelineSelectionPreviewSync.ts

@@ -1,7 +1,7 @@
 import { useEffect, useMemo } from "react";
 import type { TimelineElement } from "../player";
 import type { DomEditSelection } from "../components/editor/domEditing";
-import { findMatchingTimelineElementId, findTimelineIdByAncestor } from "../utils/studioHelpers";
+import { resolveTimelineIdForSelection } from "../utils/studioHelpers";
 
 interface UseTimelineSelectionPreviewSyncParams {
   selectedElementId: string | null;
@@ -26,21 +26,6 @@ function orderSelectedIds(ids: Set<string>, anchor: string | null): string[] {
   return [anchor, ...ordered.filter((id) => id !== anchor)];
 }
 
-function selectionTimelineId(
-  selection: DomEditSelection,
-  timelineElements: TimelineElement[],
-  activeCompPath: string | null,
-): string | null {
-  return (
-    findMatchingTimelineElementId(selection, timelineElements) ??
-    findTimelineIdByAncestor(
-      selection.element,
-      timelineElements,
-      selection.sourceFile || activeCompPath || "index.html",
-    )
-  );
-}
-
 function selectionIdsMatch(currentIds: string[], selectedIds: string[]): boolean {
   if (currentIds.length !== selectedIds.length) return false;
   const selected = new Set(selectedIds);
@@ -72,7 +57,9 @@ export function useTimelineSelectionPreviewSync({
           ? [domEditSelection]
           : [];
     const currentIds = currentSelections
-      .map((selection) => selectionTimelineId(selection, timelineElements, activeCompPath))
+      .map((selection) =>
+        resolveTimelineIdForSelection(selection, timelineElements, activeCompPath),
+      )
       .filter((id): id is string => Boolean(id));
 
     if (selectedIds.length === 0) {