[Epic] v1.3.0 — Tour

Tracking issue for the v1.2.0 release: Tour — a headless, accessible primitive for guided product tours and feature discovery.

Theme

Greenfield flagship. Every headless ecosystem explicitly punts Tour; v0 already owns the sub-parts (createStep, usePopover, useRovingFocus, observers, Scrim, Portal, useStorage). Split into two sub-issues so the genuinely-new work has its own runway ahead of the component shell:

1. createTour primitive — step orchestration, target-rect tracking, SVG-mask cutout geometry, interaction passthrough, per-step a11y.
2. Tour compound component — the surface over createTour.

Naming createTour is provisional — rename here if you'd rather fold the orchestration into the component.

Sub-issues track each against the new-feature checklist.