Add the agent heartbeat; retune listening so it stops flickering

The face now treats /hermes/state as the agent's heartbeat: when nothing has been
heard for agent_timeout_s (10 s) it logs "agent gone" and sleeps, so "sleeping"
really means "no agent connected". It also logs every state change, which is how
a missed wake message was found: a one-off publish before discovery matches is
lost under volatile QoS.

The live log showed the face flickering between listening and idle about twice a
second, because the detector let go after 0.4 s. Hold is now 1.2 s so pauses
between words do not drop it, and the margin 7 dB over the adaptive floor (quiet
room noise peaks sit about 3 dB over it), with tests for both.

Confirmed by the owner on the real display: gaze directions, and a mouth that
follows their live voice proportionally.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AFN8nRRCiDiMN7ChBpHJhs

ros/hermes_audio/test/test_levels.py

@@ -42,9 +42,9 @@ def test_listens_to_a_voice_and_lets_go_after_the_hold():
     assert a.state == 'quiet'
     t = run_activity(a, -35, -100, 0.5, t)
     assert a.state == 'listening'
-    run_activity(a, -62, -100, 0.2, t)
-    assert a.state == 'listening'                      # still holding
-    run_activity(a, -62, -100, 1.0, t + 0.2)
+    run_activity(a, -62, -100, 0.8, t)
+    assert a.state == 'listening'                      # still holding, through a pause between words
+    run_activity(a, -62, -100, 1.0, t + 0.8)
     assert a.state == 'quiet'
 
 
@@ -74,10 +74,32 @@ def test_a_room_that_is_already_noisy_calibrates_instead_of_listening_forever():
     assert a.state == 'quiet' and a.mic_threshold_db > -40
     t = run_activity(a, -25, -100, 0.5, 5.0)           # a voice 18 dB above it
     assert a.state == 'listening'
-    run_activity(a, -43, -100, 1.0, t)
+    run_activity(a, -43, -100, 2.0, t)
     assert a.state == 'quiet'
 
 
+def test_pauses_between_words_do_not_flicker():
+    a = Activity()
+    t = run_activity(a, -43, -100, 5)
+    states = []
+    for _ in range(6):                                 # six words, 0.3 s each with 0.3 s gaps
+        for level, secs in ((-25, 0.3), (-43, 0.3)):
+            for _ in range(int(secs / 0.05)):
+                t += 0.05
+                states.append(a.update(level, -100, t, 0.05))
+    assert states[8:].count('quiet') == 0              # once it starts listening it stays there
+
+
+def test_quiet_room_noise_peaks_stay_below_the_threshold():
+    a = Activity()
+    t = run_activity(a, -43, -100, 5)
+    flicker = []
+    for i in range(200):
+        t += 0.05
+        flicker.append(a.update(-43 + (3 if i % 7 == 0 else 0), -100, t, 0.05))   # peaks 3 dB over the floor
+    assert set(flicker) == {'quiet'}
+
+
 def test_a_new_steady_noise_is_absorbed_after_a_while():
     a = Activity()
     t = run_activity(a, -60, -100, 5)                  # quiet room