feat(tokens): add design tokens, type scale, push scaffold, and root nav state

lib/widgets/pluri_push_scaffold.dart

@@ -0,0 +1,85 @@
+import 'package:flutter/material.dart';
+
+import '../tema/pluriwave_theme.dart';
+import 'pluri_wave_scaffold.dart';
+
+/// Design ADR-2: chrome is decided by route topology, not a parameter.
+/// [PluriPushScaffold] is the ONE shape a second-level (pushed) screen may
+/// take — root screens are plain body widgets living in `_PaginaPrincipal`
+/// and never construct a Scaffold themselves.
+///
+/// The load-bearing part of this API is what it does NOT have: there is no
+/// `bottomNavigationBar` parameter. That absence is what makes "second-level
+/// screens have no tab bar" unrepresentable rather than a convention someone
+/// can forget.
+class PluriPushScaffold extends StatelessWidget {
+  const PluriPushScaffold({
+    super.key,
+    required this.title,
+    required this.body,
+    this.titleOverride,
+    this.leadingIcon = Icons.arrow_back_rounded,
+    this.onBack,
+    this.actions = const <Widget>[],
+    this.bottom,
+    this.floatingActionButton,
+  });
+
+  /// Styled once, here, with [PluriWaveTypography.screenTitle] — the reason
+  /// this is a String and not a Widget, so screens stop copy-pasting the
+  /// style. See [titleOverride] for the one documented exception.
+  final String title;
+
+  final Widget body;
+
+  /// Single documented exception to [title]: the full player's centered
+  /// "EN DIRECTO" pill (WU14). A second consumer means ADR-2 gets revisited,
+  /// not that this parameter quietly becomes the general case.
+  final Widget? titleOverride;
+
+  /// Defaults to a back arrow; `pantalla_reproductor.dart` dismisses with
+  /// `keyboard_arrow_down` instead — still one pushed route, one back
+  /// affordance, only the glyph differs.
+  final IconData leadingIcon;
+
+  /// Defaults to [Navigator.maybePop].
+  final VoidCallback? onBack;
+
+  final List<Widget> actions;
+
+  /// Optional persistent footer (e.g. a CTA).
+  final PreferredSizeWidget? bottom;
+
+  final Widget? floatingActionButton;
+
+  static const double headerHeight = 56;
+
+  /// Centralises route construction so the transition is uniform and a test
+  /// can assert "pushed, not index-switched". Precedent:
+  /// `PantallaReproductor.abrir`.
+  static Future<T?> push<T>(BuildContext context, WidgetBuilder builder) {
+    return Navigator.of(
+      context,
+    ).push<T>(MaterialPageRoute<T>(builder: builder));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final type = context.pluriType;
+    return PluriWaveScaffold(
+      appBar: AppBar(
+        toolbarHeight: headerHeight,
+        automaticallyImplyLeading: false,
+        leading: IconButton(
+          icon: Icon(leadingIcon),
+          onPressed: onBack ?? () => Navigator.maybePop(context),
+        ),
+        title: titleOverride ?? Text(title, style: type.screenTitle),
+        actions: actions,
+        bottom: bottom,
+      ),
+      body: body,
+      floatingActionButton: floatingActionButton,
+    );
+  }
+}