feat(ui): every app's frame around its tabs, Learn's moved into halkyon_ui

NavigationFrame is Learn's AppShell without its router and its tabs: a
navigation rail from medium windows on with the app's icon on top,
leading to the first tab, a bar floating above the foot below, the
background of the screen in view around it, and a tick on every choice.
TabsStack is Learn's container of the tabs' navigators, every tab kept
alive and told whether it shows. Learn's shell is built on both, its
tests unchanged; Train's is next.

The rail's icon is drawn from 120 px at most in memory, as Learn's was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/README.md

@@ -27,12 +27,14 @@ LeanCode's lints.
   `readableContentMaxWidth`.
 - **Widgets** (`lib/src/widget/`): `showAdaptiveSheet` with `SheetScaffold`;
   `TopGradient`, `ScreenTint`, `ScreenColor`, `TintFrame`, `FramedScreen`
-  and `TabVisibility`; `FloatingNavigationBar`; `AppLoadingIndicator`,
-  `ButtonLoadingIndicator`, `AppRefreshIndicator`; `SectionCard`,
-  `SliverCentered`, `EmptyListMessage`, `CountBadge`, `TitleWithKicker`,
-  `CollapsingRow`, `ScrollHidingFab`, `liftWhileDragged`, `ReorderToggle`,
-  `ReorderGrip`; `ChevronField`, `requiredDecoration`, `FieldInfoButton`,
-  `CountStepper`, `PaletteColorPicker`, `TextLimits` with
+  and `TabVisibility`; `FloatingNavigationBar`, and `NavigationFrame`, every
+  app's frame around its tabs (the rail with the app's icon, the floating
+  bar, a tick on every choice), whichever router drives it;
+  `AppLoadingIndicator`, `ButtonLoadingIndicator`, `AppRefreshIndicator`;
+  `SectionCard`, `SliverCentered`, `EmptyListMessage`, `CountBadge`,
+  `TitleWithKicker`, `CollapsingRow`, `ScrollHidingFab`, `liftWhileDragged`,
+  `ReorderToggle`, `ReorderGrip`; `ChevronField`, `requiredDecoration`,
+  `FieldInfoButton`, `CountStepper`, `PaletteColorPicker`, `TextLimits` with
   `nearLimitCounter`; `confirmDestructiveAction`, `showUndoSnackBar`,
   `showMoreInfoSheet`; `FoundById`; `KeyboardBackOnResume`; `BrandWordmark`
   and `LaunchScreen`, which take the app's word and icon.