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/widgets/lawheatmap_card.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/material.dart';
+
+import '../theme/lawheatmap_tokens.dart';
+
+/// Surface container — visual equivalent of fai_chain_studio's
+/// `ChainCard`, retuned to the fai_web palette.
+class LawHeatmapCard extends StatelessWidget {
+  const LawHeatmapCard({
+    super.key,
+    required this.child,
+    this.padding =
+        const EdgeInsets.all(LawHeatmapSpace.lg),
+    this.accent = false,
+    this.expand = false,
+  });
+
+  final Widget child;
+  final EdgeInsetsGeometry padding;
+
+  /// If true, draws a thin petrol accent bar at the top —
+  /// used for "this card carries a primary action".
+  final bool accent;
+
+  /// If true, the card grows to fill its parent's bounded
+  /// dimensions and forces the child to expand. Use when the
+  /// card is placed inside an `Expanded` widget and the child
+  /// is itself a free-sizing widget (e.g. CustomPaint).
+  final bool expand;
+
+  @override
+  Widget build(BuildContext context) {
+    final padded = Padding(padding: padding, child: child);
+    return Card(
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        mainAxisSize: expand ? MainAxisSize.max : MainAxisSize.min,
+        children: [
+          if (accent)
+            Container(
+              height: 2,
+              decoration: const BoxDecoration(
+                color: LawHeatmapColors.signal,
+                borderRadius: BorderRadius.vertical(
+                  top: LawHeatmapRadius.md,
+                ),
+              ),
+            ),
+          if (expand) Expanded(child: padded) else padded,
+        ],
+      ),
+    );
+  }
+}