After 30s stable at a tier, the AdaptiveQualityController actively
probes the next tier up by switching the encoder and observing for 5s.
If loss/RTT stay within the target tier's thresholds, the upgrade
commits. If >1 bad report, the probe aborts with a 60s cooldown.
Probing is disabled on cellular (studio tiers aren't classified there)
and skipped when already at Studio64k (highest tier).
This complements the passive upgrade path (10 consecutive good reports)
by actively discovering that a path can sustain higher quality, rather
than waiting for the classification to drift upward.
New: ProbeState struct, check_probe() method, 4 constants, 5 tests.
377 tests passing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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