fix(store,doctor): doc-verifier findings on the trust surfaces

The doc-verifier pass over the new trust/exposure surfaces came back
PASS with five improvements, all applied:
- the store policy notice gains a 'Learn more' into the security doc
  (the notice named security.require_signatures but not where it
  lives)
- 'blocked' disables the trust gate's install button — an active
  button contradicted the 'install would be refused' statement right
  above it (guard test added)
- the unknown-exposure tooltip now says what the operator can do
  (check where the name resolves)
- dead l10n key verifPillUnverified removed (unverified is the
  page-level notice, never a card pill)
- stale header comment in install_verification.dart corrected

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

lib/pages/store.dart

@@ -15,6 +15,7 @@ import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import '../data/error_presentation.dart';
 import '../data/hub.dart';
 import '../data/install_verification.dart';
+import 'welcome.dart' show showFaiDoc;
 import '../data/system_actions.dart';
 import '../data/today_story_loader.dart';
 import '../l10n/app_localizations.dart';
@@ -399,6 +400,12 @@ class _StorePageState extends State<StorePage> {
                             child: ChainInlineHelp(
                               icon: Icons.gpp_maybe_outlined,
                               text: l.storePolicyUnverifiedNotice,
+                              // One click to the security doc that
+                              // explains WHERE the key lives
+                              // (doc-verifier finding).
+                              onLearnMore: () =>
+                                  showFaiDoc(context, 'security'),
+                              learnMoreLabel: l.buttonLearnMore,
                             ),
                           ),
                         if (_aiThinking ||