fix(chat): release live follow on middle-button pan, Shift+Space, and nested wheel

Merge main and reshape the follow opt-out to the gestures the timeline was
missing: a middle-button press starts the platform autoscroll pan (the only
scroll gesture on wheel-less mice and tablets with a pointer), Shift+Space
scrolls up from the keyboard, and an upward wheel over a nested scroller that
still has room above stays with that scroller instead of releasing the chat.

The grace re-pin timer and scroll-direction tracking are dropped: returning to
within the end band already re-arms follow, and the mode machine is built
without timers on purpose. Pause/Break never move the viewport and are not
gestures.

Closes #1640

packages/ui/src/components/chat/lib/scroll/timelineScrollIntent.ts

@@ -0,0 +1,41 @@
+// Gesture classification for the chat timeline's follow opt-out.
+//
+// The timeline releases live follow on REAL upward gestures only. Wheel and
+// touch carry their direction; this module answers the same question for the
+// inputs that do not: which keys mean "scroll up", when a middle-button press
+// starts a pan, and when an upward wheel belongs to a nested scroller (a tool
+// output box) that can still consume it. Pure functions, no DOM ownership,
+// so the rules are testable without a renderer.
+
+// A nested scroller inside the timeline marks itself with this attribute
+// (see ToolPart). Wheel-up over it scrolls the box, not the conversation, for
+// as long as the box has room above.
+const NESTED_SCROLLABLE_SELECTOR = '[data-scrollable]';
+
+export const isFollowReleaseKey = (
+    event: Pick<KeyboardEvent, 'altKey' | 'ctrlKey' | 'key' | 'metaKey' | 'shiftKey'>,
+): boolean => {
+    // Modified keys are shortcuts, not navigation.
+    if (event.altKey || event.ctrlKey || event.metaKey) return false;
+    if (event.key === ' ') return event.shiftKey;
+    return event.key === 'ArrowUp' || event.key === 'PageUp' || event.key === 'Home';
+};
+
+const nestedScrollable = (root: HTMLElement, target: EventTarget | null): HTMLElement | null => {
+    if (!(target instanceof Element)) return null;
+    const nested = target.closest(NESTED_SCROLLABLE_SELECTOR);
+    return nested instanceof HTMLElement && nested !== root ? nested : null;
+};
+
+// An upward wheel over a nested scroller that still has content above stays
+// with that scroller; the timeline must not treat it as leaving the end.
+export const nestedScrollableConsumesWheelUp = (root: HTMLElement, target: EventTarget | null): boolean => {
+    const nested = nestedScrollable(root, target);
+    return nested !== null && nested.scrollTop > 0;
+};
+
+// Middle-button press starts the platform's autoscroll pan (Windows/Linux
+// Chromium); the pan then scrolls without wheel events, so the press itself is
+// the gesture. Inside a nested scroller the pan belongs to that scroller.
+export const isMiddleButtonPan = (root: HTMLElement, event: Pick<MouseEvent, 'button' | 'target'>): boolean =>
+    event.button === 1 && nestedScrollable(root, event.target) === null;