docs(studio): document timeline keyboard navigation (#3031)

* feat(studio): expose timeline treegrid semantics

* feat(studio): coordinate logical timeline focus

* feat(studio): add timeline keyboard controls

* docs(studio): document timeline keyboard navigation

packages/studio/src/player/components/TimelineClip.tsx

@@ -3,6 +3,7 @@ import type { TimelineElement } from "../store/playerStore";
 import { defaultTimelineTheme, getClipHandleOpacity, type TimelineTheme } from "./timelineTheme";
 import type { TimelineEditCapabilities } from "./timelineEditing";
 import { isAudioTimelineElement } from "../../utils/timelineInspector";
+import { timelineClipFocusId } from "./timelineNavigationIdentity";
 
 interface TimelineClipProps {
   el: TimelineElement;
@@ -18,6 +19,7 @@ interface TimelineClipProps {
   capabilities: TimelineEditCapabilities;
   theme?: TimelineTheme;
   isComposition: boolean;
+  tabIndex?: 0 | -1;
   onHoverStart: () => void;
   onHoverEnd: () => void;
   onPointerDown?: (e: React.PointerEvent) => void;
@@ -43,6 +45,7 @@ export const TimelineClip = memo(function TimelineClip({
   capabilities,
   theme = defaultTimelineTheme,
   isComposition,
+  tabIndex = -1,
   onHoverStart,
   onHoverEnd,
   onPointerDown,
@@ -82,19 +85,28 @@ export const TimelineClip = memo(function TimelineClip({
     zIndex: isDragging ? 20 : isSelected ? 10 : isHovered ? 5 : 1,
     // Regular cursor over clips (CapCut-style, user preference) — no grab hand.
     cursor: "default",
+    appearance: "none",
+    color: "inherit",
+    font: "inherit",
+    padding: 0,
+    textAlign: "left",
     transform: isDragging ? "translateY(-1px)" : undefined,
   };
 
   return (
-    <div
+    <button
+      type="button"
       data-clip={isGestureActor ? undefined : "true"}
       data-el-id={isGestureActor ? undefined : (el.key ?? el.id)}
+      data-timeline-focus-id={isGestureActor ? undefined : timelineClipFocusId(el.key ?? el.id)}
       data-clip-start={el.start}
       data-clip-end={el.start + el.duration}
       data-clip-hidden={el.hidden ? "true" : undefined}
       data-active={isActive ? "" : undefined}
       aria-hidden={isGestureActor ? "true" : undefined}
-      tabIndex={isGestureActor ? undefined : -1}
+      tabIndex={isGestureActor ? undefined : tabIndex}
+      aria-label={`${displayLabel}, ${startLabel} to ${endLabel} seconds`}
+      aria-pressed={isGestureActor ? undefined : isSelected}
       className={clipClassName}
       style={style}
       title={
@@ -176,6 +188,6 @@ export const TimelineClip = memo(function TimelineClip({
         </span>
       )}
       {children}
-    </div>
+    </button>
   );
 });