Add DragGesture and onLongPressGesture

[colemancda]

Adds continuous gestures: DragGesture() with .onChanged/.onEnded applied via .gesture(_:), and .onLongPressGesture(perform:).

Keeping the bridge O(1)

A drag needs two numbers per event, but the JNI surface is a fixed set of single-value entry points and growing it is the fragile part of this architecture. So the interpreter encodes "<phase>;<startX>,<startY>;<x>,<y>" (in points) through the existing string callback and Swift rebuilds a DragGesture.Value from it — no new bridge entry point.

Two real bugs the on-device pass caught

Both were invisible to unit tests and only showed up dragging a real box:

1. Self-referential drag. The demo box offsets itself by the translation, so it moves under the finger — absolute pointer positions then compensate for that motion and the translation collapses to ~0 (observed: Offset: 1, 1 for a 150px drag). Fixed by accumulating per-event deltas instead of reading absolute positions.
2. Gesture torn down mid-drag. Every onChanged writes state → re-evaluation → a fresh callback id. pointerInput was keyed on that id, so the detector — a long-running coroutine — was cancelled and restarted on the first event, and onEnded never fired. Fixed by keying on the node's stable id and reading the current callback id from a holder, so it never dispatches to a reclaimed callback. (Same generation-scoped-id hazard as the search field and .task cancellation.)

Tap and long press also share a single combinedClickable detector rather than two competing pointer handlers, which would fight over the gesture.

Verification

• swift test — 3 new tests (long press callback distinct from tap; drag payload → translation for changed/ended; malformed payload ignored), 74 total passing
• Emulator (new "Gestures" catalog screen):
  • Drag of 150×100 px → Offset: 45, 30 dp and Last drop: 45, 30. The shortfall from the raw 55×36 dp is Compose's touch slop consumed before recognition; the 1.5 ratio is preserved exactly, and onEnded reports the same final translation.
  • Tap vs long press on the same view: a tap gave Taps: 1, Long presses: 0; a ~900 ms hold then gave Taps: 1, Long presses: 1 — each fires only its own handler.

Scope

.gesture(_:) accepts DragGesture (a Gesture marker protocol is in place for future types). Magnification/rotation and gesture composition (.sequenced, .simultaneously) are not included.