feat: add grouped column headers

[Dastari]

Summary

• add first-class nested column groups with shared multi-row headings, semantic scopes, group styling, pinning, resizing, and leaf-aware reordering
• make visibility, filtering, editing, sizing, card titles, and custom-cell behavior recurse through grouped definitions
• document grouped columns, bump the package to 4.2.0, and refresh the public API snapshot and committed distribution artifacts

Why

Grouped headers previously relied on incidental TanStack rendering while wrapper features assumed every top-level definition was a leaf. This makes grouping a supported public API and keeps existing data-table features consistent for nested columns.

Validation

• corepack pnpm lint
• corepack pnpm typecheck
• corepack pnpm demo:typecheck
• corepack pnpm test:coverage — 214 tests passed
• corepack pnpm build
• corepack pnpm api:update && corepack pnpm api:check
• corepack pnpm test:consumer
• corepack pnpm demo:build
• corepack pnpm bundle:check
• git diff --check

Environment limitation

The Playwright browser suite could not launch in this host because the Chromium libnspr4.so dependency is unavailable. No browser test body ran; the browser checks should run normally in CI.