Refactor to polling-first architecture with OBSERVE as accelerator

State freshness now comes from a tiered PollScheduler over the persistent
DTLS session; OBSERVE registrations are kept as an opportunistic
acceleration layer. Behaviour is identical online vs air-gapped except
for worst-case freshness latency.

Adds three modules:
- StateCache: single source of truth, source-tagged change events
- PollScheduler: hot/warm/cold + sweep tiers, write-defer past the
  fetchback-revert window, per-window RTT/slow-poll tracking
- KeepaliveTask: CoAP empty-CON ping with consecutive-fail detection
  driving MQTT availability

Bridge publishes per-appliance diagnostic entities (Push Active, Last
Update Source, Poll Max RTT, Slow Polls, Poll Errors, Stalest Resource
Age, Last OBSERVE Age) under HA's Diagnostic section. Tier cadences
are descriptor-declared, calibrated against measured per-firmware
ceilings (dryer ~14 req/s, oven ~8 req/s via probe_poll_rate_combined.py).

Drops HEARTBEAT_INTERVAL_S in favour of the descriptor-declared sweep
tier; PING_INTERVAL_S now consumed by KeepaliveTask inside the bridge
rather than driven from main.py.

README explains the push/poll split and what happens when the appliance
is blocked from internet.

main.py

@@ -3,13 +3,15 @@
 
 One process supervises N Samsung appliances over their OCF CoAP-DTLS
 local APIs, publishing state + HA discovery to MQTT. Each appliance
-runs its own DTLS session in its own thread. MQTT is shared.
+runs its own DTLS session in its own thread, with an in-session
+PollScheduler driving state freshness and a KeepaliveTask driving
+DTLS-layer liveness. MQTT is shared.
 
 Config is env-var driven:
   * APPLIANCE_COUNT plus APPLIANCE_<n>_{CLASS,IP,OCF_PORT,TOPIC,NAME}
     define the appliances to bridge.
   * Shared keys (MQTT_*, HA_DISCOVERY_PREFIX, CERT_PATH, KEY_PATH,
-    HEALTH_INTERVAL_S, HEARTBEAT_INTERVAL_S) apply to all.
+    HEALTH_INTERVAL_S, PING_INTERVAL_S) apply to all.
 
 Reconnects on session errors per-appliance; shuts down cleanly on
 SIGINT / SIGTERM."""
@@ -146,36 +148,12 @@ def main():
                     break
         return loop
 
-    def make_heartbeat(b: PushBridge):
-        def loop():
-            while not b.stop.is_set():
-                if b.stop.wait(shared.HEARTBEAT_INTERVAL_S):
-                    break
-                b.heartbeat()
-        return loop
-
-    def make_ping(b: PushBridge):
-        def loop():
-            while not b.stop.is_set():
-                if b.stop.wait(shared.PING_INTERVAL_S):
-                    break
-                b.ping_once()
-        return loop
-
     for b in bridges:
         tag = b.app.klass
         threads.append(threading.Thread(
             target=b.run_forever, daemon=True, name=f'{tag}-session'))
         threads.append(threading.Thread(
             target=make_health(b), daemon=True, name=f'{tag}-health'))
-        if shared.HEARTBEAT_INTERVAL_S > 0:
-            threads.append(threading.Thread(
-                target=make_heartbeat(b), daemon=True,
-                name=f'{tag}-heartbeat'))
-        if shared.PING_INTERVAL_S > 0:
-            threads.append(threading.Thread(
-                target=make_ping(b), daemon=True,
-                name=f'{tag}-ping'))
 
     stopping = threading.Event()