perf(studio-server): coordinate cancelable thumbnail generation (#2720)

* perf(studio): schedule adaptive timeline thumbnails

* perf(studio): bound thumbnail decoding resources

* perf(studio): virtualize timeline thumbnail media

* perf(studio): prioritize timeline thumbnail work

* perf(studio-server): coordinate cancelable thumbnail generation

---------

Co-authored-by: Codex <codex@local>

packages/studio/src/player/lib/thumbnailPolicy.ts

@@ -0,0 +1,24 @@
+export type ThumbnailMode = "adaptive" | "hidden";
+export type ThumbnailRuntimePolicy = "follow-preference" | "force-hidden" | "legacy-default";
+
+// "Adaptive" currently means the scheduler pauses rich work while scrolling.
+// The mode name leaves room for finer-grained runtime heuristics later.
+
+const rawPolicy = import.meta.env.VITE_STUDIO_TIMELINE_THUMBNAIL_POLICY;
+
+const studioThumbnailRuntimePolicy: ThumbnailRuntimePolicy =
+  rawPolicy === "force-hidden" || rawPolicy === "legacy-default" ? rawPolicy : "follow-preference";
+
+export function defaultThumbnailMode(
+  storedMode: ThumbnailMode | undefined,
+  policy: ThumbnailRuntimePolicy = studioThumbnailRuntimePolicy,
+): ThumbnailMode {
+  return storedMode ?? (policy === "legacy-default" ? "hidden" : "adaptive");
+}
+
+export function effectiveThumbnailMode(
+  preferredMode: ThumbnailMode,
+  policy: ThumbnailRuntimePolicy = studioThumbnailRuntimePolicy,
+): ThumbnailMode {
+  return policy === "force-hidden" ? "hidden" : preferredMode;
+}