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/tema/pluriwave_tokens.dart

@@ -14,6 +14,7 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
     required this.listSurface,
     required this.liveGreen,
     required this.offlineAccent,
+    required this.balloonSurface,
     required this.radiusSm,
     required this.radiusMd,
     required this.radiusLg,
@@ -46,6 +47,12 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
   /// promote, but the hex value itself is not this file's discretion.
   final Color offlineAccent;
 
+  /// Solid capsule fill for the "barra globo" bottom navigation (Design turn
+  /// t4, option 4a — the balloon and the flat bar share this colour so their
+  /// union reads as a single inflated shape). No prior literal to promote —
+  /// new brand-system near-black distinct from [deepViolet].
+  final Color balloonSurface;
+
   final double radiusSm;
   final double radiusMd;
   final double radiusLg;
@@ -75,6 +82,7 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
     listSurface: Color(0xFF102532),
     liveGreen: Color(0xFF7EE4C2),
     offlineAccent: Color(0xFFE8879A),
+    balloonSurface: Color(0xFF0A1B24),
     radiusSm: 14,
     radiusMd: 22,
     radiusLg: 30,
@@ -95,6 +103,7 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
     Color? listSurface,
     Color? liveGreen,
     Color? offlineAccent,
+    Color? balloonSurface,
     double? radiusSm,
     double? radiusMd,
     double? radiusLg,
@@ -113,6 +122,7 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
       listSurface: listSurface ?? this.listSurface,
       liveGreen: liveGreen ?? this.liveGreen,
       offlineAccent: offlineAccent ?? this.offlineAccent,
+      balloonSurface: balloonSurface ?? this.balloonSurface,
       radiusSm: radiusSm ?? this.radiusSm,
       radiusMd: radiusMd ?? this.radiusMd,
       radiusLg: radiusLg ?? this.radiusLg,
@@ -143,6 +153,9 @@ class PluriWaveTokens extends ThemeExtension<PluriWaveTokens> {
       liveGreen: Color.lerp(liveGreen, other.liveGreen, t) ?? liveGreen,
       offlineAccent:
           Color.lerp(offlineAccent, other.offlineAccent, t) ?? offlineAccent,
+      balloonSurface:
+          Color.lerp(balloonSurface, other.balloonSurface, t) ??
+          balloonSurface,
       radiusSm: lerpDouble(radiusSm, other.radiusSm, t) ?? radiusSm,
       radiusMd: lerpDouble(radiusMd, other.radiusMd, t) ?? radiusMd,
       radiusLg: lerpDouble(radiusLg, other.radiusLg, t) ?? radiusLg,