add ClideAnchoredOverlay + ClideMenu popover primitive (D-88, T-286)

Nine surfaces hand-rolled the same anchored-overlay + row-list + barrier +
keyboard-nav pattern. Extract one owned primitive (no Material):

- ClideAnchoredOverlay (clide_anchored.dart): positioning + lifecycle —
  LayerLink/CompositedTransformFollower or centred Positioned, side/align +
  auto-flip on viewport bounds, full-screen tap-away barrier, OverlayEntry
  bookkeeping, focus capture, Esc-to-close. Driven by a ClideOverlayController.
- ClideMenu + ClideMenuListController (clide_menu.dart): a dropdown-token row
  surface (items + separators) with arrow/enter/escape nav, skip-disabled,
  active mark, per-item colour/leading glyph, keepOpenOnSelect (live-apply),
  and onArrowLeft/Right hooks. The nav controller is reusable by surfaces that
  keep bespoke rows (typeaheads, quick-open).

Additive — no call sites changed yet. D-88 records the convention (new `design`
domain): anchored pickers build on these; modal pickers stay on DialogRouter.

Tests: clide_anchored_test (open/close, barrier, Esc, centred, clean dispose)
and clide_menu_test (list-nav skip/wrap, select + onClose, disabled, Esc,
keepOpenOnSelect; pure ClideMenuListController cases).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/widgets.dart

@@ -6,6 +6,7 @@
 library;
 
 export 'src/clide_accordion.dart';
+export 'src/clide_anchored.dart';
 export 'src/clide_button.dart';
 export 'src/clide_column_hat.dart';
 export 'src/clide_code_block.dart';
@@ -14,6 +15,7 @@ export 'src/clide_filter_box.dart';
 export 'src/clide_lightbox.dart';
 export 'src/clide_markdown.dart';
 export 'src/clide_marquee.dart';
+export 'src/clide_menu.dart';
 export 'src/clide_svg_view.dart';
 export 'src/clide_toast.dart';
 export 'src/clide_icon.dart';