move daemon services to backend isolate

The merged UI/platform thread on macOS (Flutter 3.41) freezes on any
synchronous work — file I/O, Process.run, even isolate spawning during
early frames. No timing workaround (Timer, addPostFrameCallback,
Future.delayed) was reliable.

Fix: spawn a backend isolate that owns the DaemonDispatcher, GitClient,
PqlClient, FilesService, EditorRegistry, and Toolchain resolution. The
main isolate stays free for rendering. Communication uses SendPort with
the existing IPC message protocol (IpcRequest/IpcResponse/IpcEvent) —
zero new serialization.

New files:
  backend.dart       — spawns isolate, manages SendPort/ReceivePort
  backend_entry.dart — isolate entry point, boots all services
  isolate_client.dart — replaces InProcessClient for production

Toolchain now exposes resolveToolchainPaths() as a top-level function
with self-contained PATH expansion (no module-level state that would
prevent isolate message passing).

ClideTestApp gains boot-sequence tests: compute(), Isolate.run(),
sequential Process.run, and the full resolve+exec chain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/facade.dart

@@ -102,6 +102,7 @@ class KernelServices {
     List<Locale> availableLocales = const [Locale('en', 'US')],
     String? socketPath,
     DaemonClient Function(Logger, DaemonBus)? daemonClientFactory,
+    DaemonClient? isolateClient,
     bool autoStartDaemonClient = true,
     Toolchain? toolchain,
   }) async {
@@ -148,13 +149,14 @@ class KernelServices {
       settings: settings,
       toolchain: tc,
     );
-    final ipc = daemonClientFactory != null
-        ? daemonClientFactory(log, events)
-        : DaemonClient(
-            socketPath: socketPath ?? defaultSocketPath(),
-            log: log,
-            events: events,
-          );
+    final ipc = isolateClient
+        ?? (daemonClientFactory != null
+            ? daemonClientFactory(log, events)
+            : DaemonClient(
+                socketPath: socketPath ?? defaultSocketPath(),
+                log: log,
+                events: events,
+              ));
     final extensions = ExtensionManager(
       log: log,
       events: events,