[Feature] brushX: a transient drag-range mode #198

Problem

A line or area chart should keep hover tooltip and crosshair, and also let the user press-drag to get an unsnapped x range. The range should exist only while the button is held. brushX cannot do this: its range is persistent and has handles.

• range is a controlled value with no option to make it transient (interaction-brush.ts:56-66).
• Start and end handles are always built (interaction-brush.ts:216, interaction-brush.ts:226).
• A blank click commits a zero-width range (interaction-brush.ts:408-421).

Expected

An opt-in mode such as { persistent: false, handles: false }: preview events while dragging, one commit on release, then the range resets (also on pointer leave or cancel). Idle hover can use hover: 'passthrough' once #162 lands.

The docs could also link the existing application-owned route (pointer: false with interaction.resolvePointer and setControlledFocus) from the brush docs (interactions-and-selections.md:447-451, interactions-and-selections.md:370-374).

Related

PR #162 fixes hover focus being cleared while brushX is mounted. Nothing covers the transient mode.