Replace item.getBounds().contains(x, y) calls in onMouse() with a
static containsPoint() helper that checks CTabItem fields directly,
avoiding Rectangle allocation on every MouseMove, MouseDown, and
MouseUp event.

See https://github.com/eclipse-platform/eclipse.platform.swt/issues/3219

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>