[Bug] Align pointer events with Two.js coordinates and scene order

Summary

Follow up on the event system introduced in #16 and align pointer coordinates, hit ordering, bubbling, and missed-event behavior with the actual Two.js scene graph.

Known correctness gaps

• Hit testing uses top-left Two.js world coordinates, while TwoEvent.point subtracts half the canvas width and height and reports a different coordinate space.
• Shapes under different parent groups fall back to event-registration order rather than complete scene-graph draw order.
• A Group with handlers but no registered interactive child is tested through the Group bounding rectangle, which can report hits in empty areas.
• onPointerMissed currently fires on a missed pointer-up; its relationship to click, pointer-down, and pointer-capture semantics is not defined.
• Pointer capture is inferred from the renderer DOM element, but the synthetic Two.js event does not expose a clear capture API.

Two.js now has contains(...) and Group.getShapesAtPoint(...); the React event layer should use the upstream traversal where possible rather than maintain a competing partial ordering model.

Acceptance criteria

• Define and document whether event points are renderer, world, parent-local, or target-local coordinates.
• Return coordinates consistently for click, wheel, pointer, bubbling, and captured events.
• Resolve topmost targets from the complete current Two.js scene order, including nested and reordered groups.
• Define and test Group hit behavior for sparse, nested, transparent, invisible, clipped, and masked content.
• Define onPointerMissed semantics and test down/up/click/cancel sequences.
• Provide a supported pointer-capture pattern for shape dragging.
• Keep bubbling and stopPropagation() behavior deterministic.
• Add renderer coverage for SVG, Canvas, and WebGL where behavior differs.
• Add tests that assert handler calls and event payloads, not only that the renderer element exists.

Related work

• Follow-up to #16.
• Scene ordering should remain consistent with #29.
• Mask-aware interaction relates to #9.

Start by reading the event system introduced in #16 and the scene-order work in #29, then inspect the React event layer and Two.js APIs such as contains(...) and Group.getShapesAtPoint(...). Define the coordinate, hit-testing, bubbling, capture, and missed-event semantics before implementing them, and verify the acceptance criteria across SVG, Canvas, and WebGL with handler and payload assertions.