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.
