feat: make creator media edits render-safe (#3322)

* feat: make creator media edits render-safe

* fix: align media playback timing

* docs: add creator editing recipes

* docs: expand creator editing guidance

* fix: unify media source offsets

* fix: scale natural media duration

* fix: preserve natural media zero spans

* fix: align compiled natural media timing

* test: classify compiler media test as integration

* fix: drop inactive media windows

* fix: unify literal timing parsing

* fix: keep browser media parsing serializable

* fix: keep page timing readers strict

* fix: close remaining preview timing gaps

* fix(core): preserve Studio voice pitch at playback speed

* chore: keep creator contract source-neutral

packages/core/src/runtime/playbackRate.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+import { resolveNaturalMediaTimelineDuration } from "./playbackRate";
+
+function elementWith(attributes: Record<string, string>): Pick<Element, "getAttribute"> {
+  return {
+    getAttribute(name) {
+      return attributes[name] ?? null;
+    },
+  };
+}
+
+describe("resolveNaturalMediaTimelineDuration", () => {
+  it.each([
+    ["2x", 5],
+    ["0x2", 10],
+  ])("matches native playback-rate parsing for %s", (rate, expected) => {
+    expect(
+      resolveNaturalMediaTimelineDuration(elementWith({ "data-playback-rate": rate }), 10),
+    ).toBe(expected);
+  });
+
+  it.each([10, 11])("returns a known zero span at or past source EOF (start=%s)", (start) => {
+    expect(
+      resolveNaturalMediaTimelineDuration(elementWith({ "data-media-start": String(start) }), 10),
+    ).toBe(0);
+  });
+
+  it("returns null only when source duration is unknown", () => {
+    expect(resolveNaturalMediaTimelineDuration(elementWith({}), Number.NaN)).toBeNull();
+  });
+});