Replace the per-button hand-rolled LayerLink/OverlayEntry/barrier and the
bespoke MenuDropdown + MenuItemRow with the shared popover primitive (D-88).
A small _MenuOverlayAdapter bridges the single-open MenuBarController to each
button's ClideOverlayController, so one source of truth drives open/close,
hover-switch, Alt mnemonics, and Left/Right menu switching (onArrowLeft/Right).
menu_dropdown.dart and menu_item_row.dart are deleted (absorbed by ClideMenu).
Behaviour and a11y are unchanged; menu_bar_test + app_test stay green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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