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_icon.dart

@@ -15,6 +15,7 @@ class PluriIcon extends StatelessWidget {
     this.variant = PluriIconVariant.outline,
     this.size = 24,
     this.semanticLabel,
+    this.color,
   });
 
   final PluriIconGlyph glyph;
@@ -22,6 +23,12 @@ class PluriIcon extends StatelessWidget {
   final double size;
   final String? semanticLabel;
 
+  /// Explicit colour override for the Icon-fallback render path (used when
+  /// the raster asset fails to decode). `null` (default) preserves the
+  /// existing variant-driven colour for every current call site — see
+  /// `_resolveColor`.
+  final Color? color;
+
   @override
   Widget build(BuildContext context) {
     final tokens = context.pluriTokens;
@@ -82,6 +89,7 @@ class PluriIcon extends StatelessWidget {
   }
 
   Color _resolveColor(BuildContext context, PluriWaveTokens tokens) {
+    if (color != null) return color!;
     if (variant == PluriIconVariant.activeGlow) return tokens.electricMagenta;
     if (variant == PluriIconVariant.filled) {
       return Theme.of(context).colorScheme.onSurface;