Coordinates one read-only range across editor and native Text instances.
The host supplies targets in reading order and arbitrates pointer-down
against its other interactive objects. Once captured, moves may cross gaps.
A character endpoint and its squared distance from the pointer, measured in
root artboard coordinates. Offsets throughout this API are Unicode code
points, never UTF-16 code units.
A regular Text instance participating in a shared selection. Implementations
use the same layout and clipping as their renderer. Instance identity must
remain stable across frames, including repeated nested artboards.
