Release the sensor hold as soon as a valid value arrives

[michaelarnauts]

The bridge sends invalid (zero) sensor values right after we subscribe to a sensor, so we hold every value back for sensor_delay seconds. That also delays the correct values that arrive quickly — all sensors stay silent for the full delay even when the bridge has already reported real data.

Now we only hold zero values. The first non-zero value for a held sensor is treated as a real one: the hold timer is cancelled and the value is emitted immediately. Any later value for that sensor — including zero — passes straight through.

A sensor that only ever reports zero still behaves as before: the cached value is emitted when the hold expires. The timer is cancelled rather than left to fire, so _unhold_sensor doesn't re-emit the value a second time.

Tests

• test_sensor_hold now uses a zero value (it previously asserted that 100 was held, which is exactly the case that changed).
• test_sensor_hold_when_registered_after_connect reworded for the now-immediate emit.
• New test_sensor_hold_released_on_valid_value covers the early release, pass-through of later zeros, and the absence of a duplicate emit on expiry.

Full suite: 89 passed.

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