feat: rename layered views to overlay stack

[han4wluc]

Summary

• rename the layered view state and actions to overlayStack, pushOverlay, popOverlay, replaceLastOverlay, and clearOverlays
• update render-state construction, schemas, docs, specs, and VT fixtures to use overlay terminology consistently
• bump the package version from 1.2.0 to 1.3.0

Verification

• bun run build
• bun run test
• bun run lint