feat: initial F∆I Law-Heatmap Phase 0 scaffold

First end-to-end-buildable cut of the pflicht-/graph-basierte
Wirkungsanalyse von Rechtsnormen, running on Ch∆In. Demo runs
without external dependencies and without a hub connection so
the methodology + UI stack can be reviewed before any Verbands-
Kooperation or Beirats-Validierung kicks in.

Tree layout

  MACHBARKEITSSTUDIE.md       v0.3 — pflicht-basierte Methodik,
                              4-Tier-Evidenz, 8 law*-Module
                              auf Ch∆In statt der alten F∆I-
                              Plattform.
  RUN.md                      one-page how-to-run.
  flows/durchstich-gewo-14.yaml
                              vertical Phase-0 flow: pull →
                              normalize → duties → citations →
                              cohort → skm → frust → benefit →
                              attribution → system.approval.
  app/                        Flutter macOS client, design
                              language pulled from fai_web
                              (ink #08090A, paper #F4F3EF,
                              petrol signal #2E8F9E). Mock
                              repository ships five fixture
                              norms with real source URLs and a
                              permanent T4 demo-banner so no
                              figure can be mistaken for a
                              validated one.

Module repos (separate, see chain-modules*/):

  text-akoma-normalize, text-deontic-extract, graph-citation-
  extract, text-readability-score, stats-cohort-size, graph-
  shapley-attribution, econ-skm-score, law-benefit-score.

What runs today

  - flutter analyze: 0 issues
  - flutter test: 2/2 (landing + nav-to-shell smoke)
  - flutter build macos (via app/build-macos.sh) and ad-hoc
    codesign, app launches and the Dart VM service comes up
  - native cargo test green on every module
  - cargo build --release --target wasm32-wasip2 produces a
    130 KiB artefact for text-akoma-normalize

What is deliberately mock / stub

  - gRPC wire from the client to a chain serve hub (Repository
    abstraction is in place; live impl is the next step)
  - NKR Bürokratiekosten-Datenbank ingestion (for the canonical
    h-Werte that close out the Engpass per Studie §6.7)
  - DESTATIS GENESIS-API adapter for stats.cohort_size

License: Apache-2.0. Author/contact in MACHBARKEITSSTUDIE.md.
Signed-off-by: flemming-it <sf@flemming.it>

app/lib/pages/shell_page.dart

@@ -0,0 +1,91 @@
+import 'package:flutter/material.dart';
+
+import '../data/repository.dart';
+import '../theme/lawheatmap_tokens.dart';
+import '../widgets/delta_mark.dart';
+import 'about_page.dart';
+import 'heatmap_page.dart';
+import 'hub_status_page.dart';
+import 'norms_list_page.dart';
+
+/// App shell with a NavigationRail. Each rail destination is one
+/// page; pages do their own scrolling.
+class ShellPage extends StatefulWidget {
+  const ShellPage({super.key, required this.repository});
+
+  final EvaluationRepository repository;
+
+  @override
+  State<ShellPage> createState() => _ShellPageState();
+}
+
+class _ShellPageState extends State<ShellPage> {
+  int _index = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    final t = Theme.of(context).textTheme;
+    final pages = <Widget>[
+      HeatmapPage(repository: widget.repository),
+      NormsListPage(repository: widget.repository),
+      HubStatusPage(repository: widget.repository),
+      const AboutPage(),
+    ];
+
+    return Scaffold(
+      body: Row(
+        children: [
+          NavigationRail(
+            backgroundColor: Theme.of(context).colorScheme.surface,
+            extended: MediaQuery.of(context).size.width > 1100,
+            selectedIndex: _index,
+            onDestinationSelected: (i) => setState(() => _index = i),
+            leading: Padding(
+              padding: const EdgeInsets.symmetric(
+                vertical: LawHeatmapSpace.lg,
+                horizontal: LawHeatmapSpace.md,
+              ),
+              child: Column(
+                children: [
+                  const DeltaMark(size: 36),
+                  const SizedBox(height: LawHeatmapSpace.sm),
+                  Text(
+                    'F∆I',
+                    style: t.labelLarge?.copyWith(
+                      letterSpacing: 2,
+                      color: LawHeatmapColors.signal,
+                    ),
+                  ),
+                ],
+              ),
+            ),
+            destinations: const [
+              NavigationRailDestination(
+                icon: Icon(Icons.grid_view_outlined),
+                selectedIcon: Icon(Icons.grid_view),
+                label: Text('Heatmap'),
+              ),
+              NavigationRailDestination(
+                icon: Icon(Icons.list_alt_outlined),
+                selectedIcon: Icon(Icons.list_alt),
+                label: Text('Normen'),
+              ),
+              NavigationRailDestination(
+                icon: Icon(Icons.hub_outlined),
+                selectedIcon: Icon(Icons.hub),
+                label: Text('Hub'),
+              ),
+              NavigationRailDestination(
+                icon: Icon(Icons.info_outline),
+                selectedIcon: Icon(Icons.info),
+                label: Text('Methodik'),
+              ),
+            ],
+          ),
+          const VerticalDivider(width: 1),
+          Expanded(child: pages[_index]),
+        ],
+      ),
+    );
+  }
+}