test(coverage): lift os/code-block/deeplink for gate headroom

The three worst-covered files were genuinely untested, not edge cases:
os.dart 27%→~85% (inject the process runner so openURL/reveal don't spawn a
real browser), clide_code_block 39%→~90% (expose the byte→char span mapper
as a top-level fn + render tests), deeplink 29%→~75% (the confirm-opens and
not-activated paths). Buys buffer above the 95% floor so a feature batch
doesn't immediately trip the gate. 95.01% → 95.22%.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/kernel/src/os_test.dart

@@ -0,0 +1,58 @@
+import 'dart:io';
+
+import 'package:clide/kernel/src/events/bus.dart';
+import 'package:clide/kernel/src/log.dart';
+import 'package:clide/kernel/src/os.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  OsBridge bridge(OsProcessRunner run) => OsBridge(log: Logger(), events: DaemonBus(), run: run);
+
+  group('openURL', () {
+    test('runs the platform open command with the url and returns true on exit 0', () async {
+      final calls = <List<String>>[];
+      final os = bridge((exe, args) async {
+        calls.add([exe, ...args]);
+        return ProcessResult(0, 0, '', '');
+      });
+      expect(await os.openURL('https://example.com'), isTrue);
+      expect(calls.single.last, 'https://example.com');
+      expect(calls.single.first, isNotEmpty); // xdg-open / open / start
+    });
+
+    test('returns false on a non-zero exit', () async {
+      expect(await bridge((_, _) async => ProcessResult(0, 1, '', 'nope')).openURL('x'), isFalse);
+    });
+
+    test('returns false (never throws) when the runner fails', () async {
+      expect(await bridge((_, _) async => throw 'boom').openURL('x'), isFalse);
+    });
+  });
+
+  group('reveal', () {
+    test('runs the platform reveal command and returns true on exit 0', () async {
+      final calls = <List<String>>[];
+      final os = bridge((exe, args) async {
+        calls.add([exe, ...args]);
+        return ProcessResult(0, 0, '', '');
+      });
+      expect(await os.reveal('/tmp/some/file.txt'), isTrue);
+      expect(calls.single.first, isNotEmpty);
+    });
+
+    test('returns false (never throws) when the runner fails', () async {
+      expect(await bridge((_, _) async => throw 'x').reveal('/tmp/x'), isFalse);
+    });
+  });
+
+  test('fire emits an OS lifecycle event', () async {
+    final bus = DaemonBus();
+    final seen = <OsLifecycleEvent>[];
+    final sub = bus.on<OsLifecycleEvent>().listen(seen.add);
+    OsBridge(log: Logger(), events: bus).fire('resume');
+    await Future<void>.delayed(Duration.zero);
+    await sub.cancel();
+    expect(seen.single.kind, 'resume');
+    expect(seen.single.subsystem, 'os');
+  });
+}