add Dart core package — IPC envelopes, daemon, ping round-trip

First real content for the `clide` Dart package at the repo root.
One AOT-compiled binary (ADR 0005) with two modes:

  * `clide --daemon`  long-running unix-socket server; listens on
    `$XDG_RUNTIME_DIR/clide-$USER.sock` with stale-socket
    reclaim, accepts JSON-lines request/response traffic, clean
    SIGTERM shutdown unlinks the socket file.
  * `clide <subcommand>`  one-shot; opens the socket, sends a
    request, writes the response JSON to stdout, exits with the
    dispatcher's error code per ADR 0006 (0/1/2/3/4). Unknown
    subcommands forward to the daemon so extensions can register
    their own without CLI changes.

Tier 0 handlers: `ping` (returns pong + version + UTC ts) and
`version`. Both are covered by `test/ipc/` + `test/daemon/`; the
subprocess test builds `bin/clide`, starts it, pings it, SIGTERMs
it, and asserts the socket file disappears.

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

lib/src/daemon/dispatcher.dart

@@ -0,0 +1,48 @@
+import 'package:clide/clide.dart' show clideVersion;
+import 'package:clide/src/ipc/envelope.dart';
+import 'package:clide/src/ipc/schema_v1.dart';
+
+typedef CommandHandler = Future<IpcResponse> Function(IpcRequest req);
+
+class DaemonDispatcher {
+  DaemonDispatcher() {
+    register('ping', _ping);
+    register('version', _version);
+  }
+
+  final Map<String, CommandHandler> _handlers = {};
+
+  void register(String cmd, CommandHandler handler) {
+    _handlers[cmd] = handler;
+  }
+
+  Future<IpcResponse> dispatch(IpcRequest req) async {
+    final h = _handlers[req.cmd];
+    if (h == null) {
+      return IpcResponse.err(
+        id: req.id,
+        error: IpcError(
+          code: IpcExitCode.notFound,
+          kind: IpcErrorKind.notFound,
+          message: 'unknown command: ${req.cmd}',
+          hint: 'run `clide --help` for the surface.',
+        ),
+      );
+    }
+    return h(req);
+  }
+
+  Future<IpcResponse> _ping(IpcRequest req) async => IpcResponse.ok(
+        id: req.id,
+        data: {
+          'pong': true,
+          'ts': DateTime.now().toUtc().toIso8601String(),
+          'version': clideVersion,
+        },
+      );
+
+  Future<IpcResponse> _version(IpcRequest req) async => IpcResponse.ok(
+        id: req.id,
+        data: {'version': clideVersion},
+      );
+}