chore(studio): flutter_markdown_plus migration + integration-test scaffold

Two follow-ups to the May-2026 trust pass.

flutter_markdown_plus migration:

- pubspec swaps `flutter_markdown ^0.7.7` (discontinued
  upstream) for `flutter_markdown_plus ^1.0.3`, the
  actively-maintained fork. API surface
  (MarkdownStyleSheet, Markdown, MarkdownBody) is
  unchanged — the four import sites in welcome, store,
  flow_output, and theme.dart get an updated package
  string and that's it.
- All Studio analyzer + unit-test suites stay green.

Integration test scaffold:

- New `test/integration/hub_fixture.dart` boots a real
  `fai serve` subprocess on a free port against a temp
  FAI_DATA_DIR, polls until Healthy, exposes a ready
  HubClient. Idempotent teardown wipes the temp dir.
- Resolves the `fai` binary from PATH first, then from
  `../fai_platform/target/release/fai`. When neither
  exists, the fixture calls `markTestSkipped` with a
  clear message — fresh checkouts don't fail.
- One canonical test in `capabilities_test.dart` asserts
  on the bug class the May trust pass surfaced: that
  `system.approval` appears in `list_capabilities` with
  `kind=builtin` so Studio's missing-deps check never
  tries to install it. Plus a contract-shape test that
  every cap's `kind` is one of the three known wire
  values.
- README documents the cold-start gotcha (first
  `fai serve` per machine takes ~30s to build the
  curated-model DB) plus the manual warmup recipe.

Not in CI yet — wiring needs the platform build job to
publish `fai` as a CI artifact for downstream consumption.
Deferred until enough integration tests exist to justify
the CI minutes.

Signed-off-by: flemming-it <sf@flemming.it>
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/integration/capabilities_test.dart

@@ -0,0 +1,73 @@
+// First integration test against a real `fai serve` subprocess.
+//
+// What this test covers (the regression bait):
+//
+//   When the operator opens a flow that references `system.approval`
+//   or any other built-in / federated capability, the missing-deps
+//   check in Studio's Flows page must consider those caps already
+//   available — otherwise the Run button stays disabled forever
+//   and the operator's only recourse is to install a module that
+//   doesn't exist.
+//
+// Why this test as the canonical first scaffold:
+//
+//   It's the exact bug class that survived three commits of the
+//   May-2026 trust pass before someone noticed. A real-Hub test
+//   for it makes regressions impossible to ship.
+//
+// Cost: ~2 seconds (Hub start) + ~0.1 seconds (gRPC). Acceptable.
+//
+// Run manually with:
+//
+//   flutter test test/integration/
+//
+// CI does not yet run these by default — they need a built `fai`
+// binary in PATH or in ../fai_platform/target/release/. Wire that
+// in via .forgejo/workflows/ci.yml once the platform CI publishes
+// a Linux binary as an artifact for downstream jobs to consume.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'hub_fixture.dart';
+
+void main() {
+  group('Hub capability listing', () {
+    late HubFixture? fixture;
+
+    setUp(() async {
+      fixture = await HubFixture.start();
+    });
+
+    tearDown(() async {
+      await fixture?.dispose();
+    });
+
+    test('system.approval shows up as a builtin capability', () async {
+      final f = fixture;
+      if (f == null) return; // skipped — no binary
+      final caps = await f.client.listCapabilities();
+      final approval = caps.where(
+        (c) => c.capability == 'system.approval',
+      ).toList();
+      expect(approval, isNotEmpty,
+          reason: 'system.approval must appear in list_capabilities '
+              'so Studio does not try to install it through the store');
+      expect(approval.first.kind, 'builtin',
+          reason: 'kind=builtin lets Studio gate the Install button correctly');
+    });
+
+    test('every capability carries a kind tag', () async {
+      final f = fixture;
+      if (f == null) return;
+      final caps = await f.client.listCapabilities();
+      expect(caps, isNotEmpty,
+          reason: 'a fresh hub has at least the system.approval builtin');
+      for (final c in caps) {
+        expect(['wasm', 'builtin', 'federated'], contains(c.kind),
+            reason: 'cap ${c.capability} has unrecognised kind="${c.kind}" — '
+                'Studio gates the Install button on this string, so the wire '
+                'value must stay in the known set');
+      }
+    });
+  });
+}