T-128: delete IsolateClient / Backend / backend_entry.dart

Fifth slice of T-99. After T-127 the socket-loopback DaemonClient
is the only IPC path; the isolate-backed third implementation
(IsolateClient + Backend + backend_entry.dart) was never wired
through and has no remaining references. Removed wholesale; the
single service-registration site lives in main.dart's
buildDispatcher.

flutter analyze + the kernel and ipc suites stay green.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/ipc/isolate_client.dart

@@ -1,71 +0,0 @@
-/// IPC client that sends requests to a backend isolate via SendPort.
-///
-/// Replaces [InProcessClient] for production use. The backend isolate
-/// owns the [DaemonDispatcher] and all subprocess/file-I/O services.
-/// Requests and responses travel as serialized Maps over SendPort,
-/// reusing the existing IPC protocol (IpcRequest/IpcResponse/IpcEvent).
-library;
-
-import 'dart:async';
-import 'dart:isolate';
-
-import 'package:clide/clide.dart';
-import 'package:clide/kernel/src/events/bus.dart';
-import 'package:clide/kernel/src/events/types.dart';
-import 'package:clide/kernel/src/ipc/client.dart';
-
-class IsolateClient extends DaemonClient {
-  IsolateClient({
-    required super.log,
-    required super.events,
-    required SendPort backendPort,
-  })  : _backendPort = backendPort,
-        _events = events,
-        super(socketPath: '');
-
-  final SendPort _backendPort;
-  final DaemonBus _events;
-
-  /// The event bus that receives events from the backend.
-  DaemonBus get events => _events;
-  final Map<String, Completer<IpcResponse>> _pending = {};
-  int _nextId = 0;
-
-  /// Called by [Backend] to feed incoming messages from the backend isolate.
-  void handleMessage(Map<String, Object?> msg) {
-    final type = msg['type'] as String?;
-    switch (type) {
-      case 'response':
-        final resp = IpcResponse.fromJson(msg);
-        final c = _pending.remove(resp.id);
-        if (c != null && !c.isCompleted) c.complete(resp);
-      case 'event':
-        final evt = IpcEvent.fromJson(msg);
-        _events.emit(DaemonEvent(
-          subsystem: evt.subsystem,
-          kind: evt.kind,
-          data: evt.data,
-          ts: evt.timestamp,
-        ));
-    }
-  }
-
-  @override
-  bool get isConnected => true;
-
-  @override
-  Future<void> start() async {}
-
-  @override
-  Future<void> stop() async {}
-
-  @override
-  Future<IpcResponse> request(String cmd, {Map<String, Object?> args = const {}}) {
-    final id = '${_nextId++}';
-    final req = IpcRequest(id: id, cmd: cmd, args: args);
-    final c = Completer<IpcResponse>();
-    _pending[id] = c;
-    _backendPort.send(req.toJson());
-    return c.future;
-  }
-}