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/ipc/schema_v1.dart

@@ -0,0 +1,21 @@
+/// IPC wire schema version.
+///
+/// Bumped when the envelope shape changes in a non-backwards-compatible
+/// way. The daemon and app both carry this constant and reject messages
+/// whose `v:` doesn't match.
+const int ipcSchemaVersion = 1;
+
+abstract class IpcExitCode {
+  static const int ok = 0;
+  static const int userError = 1;
+  static const int toolError = 2;
+  static const int notFound = 3;
+  static const int conflict = 4;
+}
+
+abstract class IpcErrorKind {
+  static const String userError = 'user_error';
+  static const String toolError = 'tool_error';
+  static const String notFound = 'not_found';
+  static const String conflict = 'conflict';
+}