Replace rowGrouper with getRowGroupKey, rename groupIdGetter to getGroupId - #4159

Rename `rowKeyGetter` to `getRowKey`

The `DataGrid` and `TreeDataGrid` prop that returns a row's key is now
named `getRowKey`, following the `getX` convention that #4159 started
with `getRowGroupKey` and `getGroupId`. `rowKeyGetter` was the last
`xGetter` prop, and 7.0.0 is the time to rename it. The function's type
and behavior don't change.

### Breaking change

```diff
- <DataGrid rowKeyGetter={getRowKey} … />
+ <DataGrid getRowKey={getRowKey} … />
```

Affects every grid with row selection or keyed rows, in both `DataGrid`
and `TreeDataGrid`. The function's signature is unchanged, and the error
thrown when selection is used without it names the new prop.

### Changes

- Renamed the prop in `DataGridProps` (inherited by
`TreeDataGridProps`), and the internal names that follow it:
`rawRowKeyGetter` → `rawGetRowKey` in `TreeDataGrid`, and
`assertIsValidKeyGetter` → `assertIsValidGetRowKey`, whose error now
reads "Please specify the getRowKey prop to use selection".
- Rewrote the prop's JSDoc as short sentences: a function to get the key
of each row, required for row selection, recommended otherwise because
the key is also used as the `key` prop of the row elements, with a
`@returns` tag for the uniqueness requirement. The README paragraph
mirrors the first two sentences, and its callouts keep the
recommendation and the memoization advice.
- Replaced the module-level `rowKeyGetter` functions in the website
demos, browser tests, and visual tests, which all returned `row.id`,
with one generic `getRowKey` helper per tree: `website/utils.tsx` for
the demos and `test/browser/utils.tsx` for the tests. Its key type is
inferred from the row type, so the grids' `K` generic is unaffected;
`rowSelection.test.tsx` spies on `getRowKey<number>`, and the inline
`(row) => row.id` in `renderCell.test.tsx` uses the helper too.
- Renamed the README mentions, examples, and the prop's heading. No
README links pointed at the old heading's anchor.
- Apart from the shared helpers, the diff is a pure rename. #4240 and
#4236 also touch code that reads this prop; whichever lands second needs
a mechanical rebase.
- The prop stays optional, row indexes remain the fallback key, and the
`K` generic stays.

### Verification

`vp run ci` passed in full on the final commit. An earlier run had one
Firefox click timeout in `TreeDataGrid.test.tsx` ("cell navigation in a
treegrid") under machine load, and that file passed when rerun alone
with `vp test test/browser/TreeDataGrid.test.tsx --project "*firefox*"`.
No tests were added; the existing selection and key tests cover the
renamed prop. After `vp pack`, a scratch consumer importing from
`lib/index.js` type-checked `getRowKey` on `DataGrid` and
`TreeDataGrid`, and a `@ts-expect-error` confirmed that `rowKeyGetter`
is rejected, both with and without `exactOptionalPropertyTypes`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

…Row` (#4263)

Since #4159, `getRowGroupKey` and `getGroupId` return strings, so
`TreeDataGrid` only ever produces string group keys and IDs, but the
types still said `unknown`. Apps had to cast `groupKey as string`, or
keep `expandedGroupIds` in a `ReadonlySet<unknown>` that could hold
values the grid never produces. This types them all as `string`, and
exports `GroupRow`, which already appeared in public types
(`RenderGroupCellContentProps.row`, `RowHeightArgs`) but couldn't be
named.

This is the first PR of a stack of four: #4264 builds on it, and #4272
renames `GroupRow` to `RowGroup`.

### Breaking change

```diff
- const [expandedGroupIds, setExpandedGroupIds] = useState((): ReadonlySet<unknown> => new Set());
+ const [expandedGroupIds, setExpandedGroupIds] = useState((): ReadonlySet<string> => new Set());
```

`expandedGroupIds`, `onExpandedGroupIdsChange`, `GroupRow.groupKey`,
`GroupRow.parentId`, and `RenderGroupCellContentProps.groupKey` are
typed with `string` instead of `unknown`, which is what `getRowGroupKey`
and `getGroupId` produce. `GroupRow` is now exported.

### Changes

- `GroupRow.parentId` is `string | undefined` (`undefined` on the first
level) and `GroupRow.groupKey` is `string`; every `GroupRow` member now
has JSDoc, checked against `expandGroups` and `setStartRowIndexes`.
- `RenderGroupCellContentProps.groupKey` is `string`; #4264 then removes
it in favor of `row.groupKey`.
- The internal `toggleGroup` parameters of `TreeDataGrid`, `GroupedRow`,
and `GroupCell` are `string`, and `renderToggleGroup` no longer casts
`groupKey as string`.
- `GroupRow` is exported from `src/index.ts`; its README entry is no
longer "(internal)" and sits next to `RenderGroupCellContentProps`, and
the `RenderGroupCellContentProps` and `RowHeightArgs` entries link to
it.
- The `ReadonlySet<unknown>` state in the row grouping demo and in
`TreeDataGrid.test.tsx` is now `ReadonlySet<string>`. The `new Set()`
literals passed to `expandedGroupIds` infer `Set<string>` and need no
change.

### Verification

`vp run ci` passes. No tests are added, because the existing code
already type-checks the new types: `setup()` in `TreeDataGrid.test.tsx`
and the row grouping demo pass `ReadonlySet<string>` state and its
setter to the grid, `toggleGroup` passes `new Set(expandedGroupIds)` to
`onExpandedGroupIdsChange`, and `renderToggleGroup` renders `groupKey`
without a cast. A consumer type-check of the built `lib/index.d.ts`,
with and without `exactOptionalPropertyTypes`, confirms that
`ReadonlySet<unknown>` state is a type error at `expandedGroupIds` and
that `ReadonlySet<string>` state accepts the `Set<string>` from
`onExpandedGroupIdsChange`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>