feat(nav): rebuild the bottom bar as the prototype's balloon bar

The functional redesign never touched pluri_bottom_navigation.dart, so the
bar kept the old glass + magenta/coral language while every other surface
moved on. Rebuilds it to prototype t4/4a: a 52px opaque #0A1B24 pill with a
110x74 balloon raised over the active tab, a teal radial glow behind it, the
active item lifted 15px with its label, and inactive items reduced to a 23px
icon at 46% opacity.

The five tabs and their PluriIconGlyph icons are unchanged, per the standing
decision — only the bar's shape, colour and behaviour move.

altura and PluriLayout.bottomChromeInset are both asserted against the real
laid-out height rather than hardcoded guesses.

lib/widgets/pluri_layout.dart

@@ -2,13 +2,22 @@
 
 import '../tema/pluriwave_theme.dart';
 import 'mini_reproductor.dart';
+import 'pluri_bottom_navigation.dart';
 
 abstract final class PluriLayout {
   static const double horizontal = 16;
   static const double sectionGap = 12;
   static const double panelGap = 12;
   static const double compactGap = 8;
-  static const double bottomChromeInset = 146;
+
+  /// `app.dart`'s `bottomNavigationBar` stacks `MiniReproductor` above
+  /// `PluriBottomNavigation` with no gap between them, wrapped in a
+  /// `SafeArea(minimum: EdgeInsets.only(bottom: compactGap))` — this is that
+  /// same sum, derived from the two chrome widgets' own measured heights
+  /// instead of a guessed constant. See
+  /// `pluri_bottom_navigation_test.dart`'s composed-chrome-height assertion.
+  static const double bottomChromeInset =
+      MiniReproductor.altura + PluriBottomNavigation.altura + compactGap;
 
   /// ADR-7(b): `bottomChromeInset` assumes `MiniReproductor` is visible.
   /// Escuchar hides it (design's one exception — the embedded hero already