perf(daemon): back off the monitor poll when idle

The daemon monitor thread drained output and then slept a fixed short
interval on every tick, waking several times a second for the life of
the node even long after startup, when the node emits almost nothing.
Stopping it also meant waiting out the current sleep before it saw the
stop flag.

- The monitor now waits on a condition variable instead of a plain
  sleep, so a stop request wakes it immediately rather than after the
  interval. The stop flag is set under the monitor's mutex so the wake
  cannot be lost.
- The poll interval stays fast for the first 30 s after the thread
  starts and for 3 s after any output, and otherwise doubles per idle
  tick up to a 500 ms cap. Any new output drops it straight back to
  fast, so responsiveness during startup, rescan and shutdown is
  unchanged while a quiet node is polled far less often. The interval
  choice is a pure function.

Tests cover the interval function across the warmup window, the
recent-data window, the reset on new output, the doubling, and the cap.

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

tests/test_phase4.cpp

@@ -6071,6 +6071,24 @@ void testOutputSinceAcrossTrim()
     EXPECT_TRUE(got.empty());
 }
 
+void testNextMonitorInterval()
+{
+    using dragonx::daemon::EmbeddedDaemon;
+    using ms = std::chrono::milliseconds;
+    const ms fast(100), cap(500);
+    auto next = [&](ms cur, bool got, ms sinceStart, ms sinceData) {
+        return EmbeddedDaemon::nextMonitorInterval(cur, got, sinceStart, sinceData, fast, cap);
+    };
+    EXPECT_TRUE(next(fast, false, ms(1000), ms(1000)) == fast);        // first 30 s stay fast
+    EXPECT_TRUE(next(cap, false, ms(29999), ms(60000)) == fast);
+    EXPECT_TRUE(next(cap, false, ms(60000), ms(2999)) == fast);        // recent data stays fast
+    EXPECT_TRUE(next(cap, true, ms(60000), ms(60000)) == fast);        // data resets
+    EXPECT_TRUE(next(fast, false, ms(60000), ms(10000)) == ms(200));   // doubling
+    EXPECT_TRUE(next(ms(200), false, ms(60000), ms(10000)) == ms(400));
+    EXPECT_TRUE(next(ms(400), false, ms(60000), ms(10000)) == cap);    // capped at 500
+    EXPECT_TRUE(next(cap, false, ms(60000), ms(10000)) == cap);
+}
+
 void testOutputWindowSaturation()
 {
     using dragonx::daemon::appendBounded;
@@ -18694,6 +18712,7 @@ int main()
     testShouldConfirmStop();
     testOutputSinceAcrossTrim();
     testOutputWindowSaturation();
+    testNextMonitorInterval();
     testOutputEndWatermarkReplay();
     testChooseDaemonStatus();
     testDaemonStopStringsTranslated();