* feat!: decouple selection from DOM focus in TreeView and GridView

Move focus and blur calls out of the selected setter in TreeViewItem
and GridViewItem so that selection is pure state with no DOM focus
side effects. Focus is now handled explicitly by the interaction
handlers (keyboard arrow keys) that need it, while click interactions
rely on the browser native focus behavior.

This follows the convention used by React Aria, Angular CDK and other
major component libraries where selection state and DOM focus are
independent concerns. It eliminates focus-stealing when selection is
changed programmatically (e.g. syncing a tree panel with a grid panel).

BREAKING CHANGE: Setting selected on TreeViewItem or GridViewItem
no longer moves DOM focus. Consumers that relied on this implicit
behavior must call item.focus() explicitly after setting selection.

Made-with: Cursor

* test: add focus/selection decoupling tests for TreeView and GridView

Verify that setting selected programmatically does not move DOM focus,
and that arrow key navigation still moves both selection and focus.

Made-with: Cursor

* test: expand focus tests to cover all arrow key directions

TreeView: add ArrowUp, ArrowRight (into child), ArrowLeft (to parent),
and ArrowLeft collapse (stays focused without moving selection).
GridView: add ArrowLeft navigation.

Made-with: Cursor

* fix: remove unused variable in ArrowRight focus test

Made-with: Cursor

* fix: use treeView.dom directly instead of fragile root.dom.parentElement in test

Made-with: Cursor

* feat: implement roving tabindex for TreeView and GridView

Each widget now acts as a single Tab stop with internal arrow key navigation (WAI-ARIA roving tabindex pattern). On keyboard Tab-in, the focused item is auto-selected using :focus-visible detection. A :focus-visible CSS rule mirrors the selected style so the active item is always visually apparent. Adds a tabindex.html example for manual testing.

Made-with: Cursor

* Update src/components/GridViewItem/style.scss

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* test: add roving tabindex tests for TreeView and GridView

Covers single tabIndex=0 invariant, active item tracking selection, active item reassignment on removal, and focus-in-from-outside auto-selection. Also fixes GridView _onRemoveGridViewItem to find the next active item from remaining DOM children rather than the detached item's siblings.

Made-with: Cursor

* Update src/components/TreeView/index.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix: preserve multi-selection on keyboard Tab-in for GridView

Matches the TreeView fix: only select the focused item if it is not already selected, avoiding collapse of an existing multi-selection when tabbing back into the widget.

Made-with: Cursor

* fix: ensure _ensureActiveItem skips hidden and filtered-out items

During filtering, non-filter-result items are now skipped. Outside filtering, explicitly hidden items are skipped. Uses item.hidden instead of offsetParent for jsdom compatibility.

Made-with: Cursor

* Update test/components/treeview.mjs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>