perf(miner): abort xmrig stat fetches on stop

The miner's stats and pool-API fetches are blocking HTTP calls on the
monitor thread with multi-second timeouts. When the miner was stopped,
stop() still had to wait out an in-flight request before the monitor
thread could join.

- Both fetches now install a curl progress callback that aborts the
  transfer as soon as a stop is requested, so a request in flight ends
  promptly instead of running down its timeout.
- An abort triggered by stop is treated as a clean cancellation, not a
  fetch failure, so it is not logged or counted against the failure
  warning.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/daemon/xmrig_manager.h

@@ -157,6 +157,9 @@ private:
     // Set last_error_ under error_mutex_ (writers run on both the main thread and the monitor thread).
     void setLastError(std::string e) { std::lock_guard<std::mutex> lk(error_mutex_); last_error_ = std::move(e); }
     void fetchStatsHttp();  // Blocking HTTP call — runs on monitor thread only
+    // curl xfer-info callback: abort an in-flight stats fetch once stop() is requested.
+    static int abortCb(void* p, int64_t, int64_t, int64_t, int64_t)
+    { return static_cast<const XmrigManager*>(p)->should_stop_.load() ? 1 : 0; }
     void fetchPoolApiStats();  // Fetch pool-side stats (hashrate) from pool HTTP API
 
     std::atomic<State> state_{State::Stopped};