fix(daemon): supervise a daemon restarted after a crash

After an unexpected exit the monitor thread returned on its own but
stayed joinable, and start() only spawned a monitor when none was
joinable. A node restarted after a crash (no stop() in between) then ran
unwatched: a second crash was never detected, its output was never
drained, and the dead run's stdout pipe leaked into the next fork.

- start() joins a monitor that returned on its own (only when joinable,
  at the top, after the "already running" early returns), closes the
  dead run's stdout pipe (Windows: process and pipe handles), and always
  starts a new monitor for the process it launched. crash_count_ is kept.
- New lifecycle_mutex_ serializes start(), beginStop(), stop()'s flag
  writes and joins, and endStop(), so two threads can never join the same
  monitor. The monitor thread and the state callback never take it; a
  beginStop() racing a start() waits for it to return (its longest wait
  is the ~300 ms datadir-lock poll).
- start() is refused (returns false, state unchanged) while a stop is in
  progress, i.e. between beginStop() and endStop().
- The daemon tests now start the stand-in only in isolation (temp
  -datadir override, port check skipped): they no longer write the real
  ~/.hush/DRAGONX/obsidiandragon-dragonxd.pid or tail the real debug.log.
  New tests: restart after a crash is supervised, a restarted run stops
  cleanly, no fd leak over crash+restart cycles (Linux), and start() is
  refused while stopping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/daemon/embedded_daemon.h

@@ -53,7 +53,8 @@ public:
     /**
      * @brief Start the embedded dragonxd daemon
      * @param binary_path Path to dragonxd binary (auto-detect if empty)
-     * @return true if process started successfully
+     * @return true if process started successfully (or is already running); false while a stop
+     *         session (beginStop() .. endStop()) is still in progress
      */
     bool start(const std::string& binary_path = "");
     
@@ -501,6 +502,9 @@ private:
     std::thread monitor_thread_;
     std::atomic<bool> should_stop_{false};
     std::atomic<bool> stop_requested_{false};  // a stop was asked for: the next exit is not a crash
+    // Serializes start/beginStop/stop/endStop over monitor_thread_, should_stop_ and stop_requested_ (two
+    // threads joining one std::thread is undefined). Never taken by the monitor thread or the state callback.
+    std::mutex lifecycle_mutex_;
     std::set<std::string> debug_categories_;
     int max_connections_ = 0;  // 0 = daemon default
     bool stratum_enabled_ = false;   // -stratum: host a RandomX pool (v1.3.0+; older daemons ignore it)