Add AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+## Quick ref
+
+```shell
+npm install                  # setup (requires Node.js ≥ 22 for `node --run`)
+node --run build             # library → lib/
+node --run typecheck         # tsc --build
+node --run check             # biome check (warnings are errors)
+node --run eslint            # eslint --max-warnings 0
+node --run eslint:fix        # eslint --fix
+node --run format            # oxfmt
+node --run test              # vitest (browser + node)
+node --run test -- <path>    # single test, e.g. test/browser/rowHeight.test.ts
+```
+
+## Architecture
+
+react-data-grid is a data grid with **zero `dependencies`** (peer dependency: React 19.2+). It uses CSS Grid for layout and implements row/column virtualization in JS.
+
+```text
+src/
+  index.ts            # public API surface; all exports go through here
+  DataGrid.tsx        # main <DataGrid> component (generic: <R, SR, K>)
+  TreeDataGrid.tsx    # wraps DataGrid, adds row grouping (role="treegrid")
+  types.ts            # shared type definitions (e.g. Column, CalculatedColumn, render props, events)
+  hooks/              # shared custom React hooks
+  utils/              # pure utilities (e.g. keyboard, DOM, events, colSpan, style)
+  style/              # build-time CSS via ecij tagged templates; layers.css declares @layer order
+  cellRenderers/      # default cell renderers (e.g. checkbox, toggleGroup, value)
+  editors/            # default editors (renderTextEditor)
+test/
+  browser/            # vitest browser-mode tests (Playwright, Chromium + Firefox)
+  node/               # vitest SSR tests (Node.js)
+  visual/             # vitest visual regression tests (CI-only — never run locally)
+website/              # demo site (Vite + TanStack Router)
+```
+
+## Conventions
+
+- **Public API** — all exports flow through `src/index.ts`. Keep `README.md` in sync with user-facing changes.
+- **Docs** — keep `AGENTS.md` in sync with tooling, conventions, or architectural changes.
+- **Default renderers** — `DataGridDefaultRenderersContext` allows overriding default renderers (`renderCheckbox`, `renderSortStatus`, `renderRow`, `renderCell`, `noRowsFallback`) without prop-drilling.
+- **TypeScript strict** with `exactOptionalPropertyTypes`, `verbatimModuleSyntax`, `erasableSyntaxOnly`. Distinguish missing properties from `undefined` values.
+- **`Maybe<T>`** (`T | undefined | null`) — used for all nullable column/render props. Do not use bare `T | undefined`.
+- **`NoInfer<>`** — wrap callback parameters to prevent reverse type inference into component generics.
+- **CSS layers** — all styles live in nested `@layer rdg.<Name>` sub-layers (e.g. `rdg.Cell`, `rdg.Row`; declared in `src/style/layers.css`). Use `ecij` `css` tagged templates (build-time extraction, not runtime CSS-in-JS). Co-locate styles in component files; `src/style/` is for shared styles.
+- **Dual classnames** — components apply both a semantic class (`rdg-cell`) and a generated hash. Preserve both.
+- **Light/dark mode** — handled via CSS `light-dark()` + `color-scheme`, not JS.
+- **Accessibility first** — ARIA attributes (e.g. `aria-colindex`, `aria-rowindex`, `aria-selected`, roles) are required. Tests query by role.
+- **Formatting** — oxfmt (not Prettier). **Linting** — Biome + ESLint (both must pass with zero warnings).
+- **Build** — Rolldown bundles library to `lib/`; `ecij` plugin prefixes classes with `rdg-{version}-` (dots→dashes) to avoid cross-version conflicts.
+
+## Testing
+
+- Browser tests use `vitest/browser` + Playwright. `test/setupBrowser.ts` configures `page.render()` via `vitest-browser-react` and registers custom locators via `locators.extend()` — prefer `page.getGrid()`, `page.getCell({ name })`, `page.getRow()`, `page.getHeaderCell()`, `page.getSelectedCell()`, etc. over raw `page.getByRole()`.
+- Test helpers in `test/browser/utils.tsx`: `setup()`, `getRowWithCell()`, `getCellsAtRowIndex()`, `validateCellPosition()`, `scrollGrid()`, `tabIntoGrid()`, `testCount()`, `testRowCount()`.
+- `test/failOnConsole.ts` fails tests on unexpected console warnings/errors.
+- **Never run visual regression tests locally** — screenshots are CI-only and environment-dependent.
+
+## Validation
+
+Run before submitting changes: `node --run typecheck`, `node --run check`, `node --run eslint`, `node --run format`, `node --run test`.