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>

test/ipc/paths_test.dart

@@ -0,0 +1,28 @@
+import 'dart:io';
+
+import 'package:clide/clide.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('defaultSocketPath', () {
+    final originalXdg = Platform.environment['XDG_RUNTIME_DIR'];
+    final originalUser = Platform.environment['USER'];
+
+    test('uses XDG_RUNTIME_DIR when set', () {
+      // We can't mutate Platform.environment from dart:io, so this test
+      // just asserts the path shape for the current env. CI and dev
+      // boxes both have meaningful USER values.
+      final path = defaultSocketPath();
+      expect(path, endsWith('.sock'));
+      expect(path, contains('clide-'));
+      if (originalXdg != null && originalXdg.isNotEmpty) {
+        expect(path, startsWith(originalXdg));
+      } else {
+        expect(path, startsWith('/tmp'));
+      }
+      if (originalUser != null && originalUser.isNotEmpty) {
+        expect(path, contains('clide-$originalUser'));
+      }
+    });
+  });
+}