refactor(ui,learn,todo,train,meditate): the navigation bar's one look, the debug section gone

The developer decided on 8 Nov 2026, after trying the variants on the
phone: the wave in the colour of the screen in view, without Material's
ripple, the indicator kept and there at once, a 1.2 s wave, top corners
of 20 dp, the bar's own neutral surface. Learn's bar stays cobalt, the
colour its tabs wash in, rather than its icon's purple.

So the bar has one look: FlushNavigationBar.waveDuration and
cornerRadius are constants, and the debug section goes from the four
Settings screens with what it switched (NavigationBarDebugSection,
debugNavigationBarStyle, effectiveNavigationBarStyle, the style record
and the variants A, C and F, the speeds, the corners and the tinted
surface). Their tests go too; those stating the look kept stay. Learn's
sign-out helper no longer has to scroll past the section.

ADR 0030's amendment of D60, the halkyon_ui README and Learn's
AGENTS.md record the decision and the alternatives set aside.

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

apps/learn/halkyon_learn_flutter/test/feature/auth/widget/session_widgets_test.dart

@@ -47,9 +47,6 @@ void main() {
   /// Signs out from Settings, which asks how to start.
   Future<void> signOut(WidgetTester tester, {bool passwords = true}) async {
     await openSettings(tester, passwords: passwords);
-    // Below the debug builds' section of the navigation bar's looks.
-    await tester.ensureVisible(find.text('Sign out'));
-    await tester.pumpAndSettle();
     await tester.tap(find.text('Sign out'));
     await tester.pumpAndSettle();
   }