When SetMaxSpatialLayer() is called with target/current layers in
InvalidLayerSpatial state, opportunistically initialize the target
layer to avoid dropped packets during async stream allocator
initialization.
Guards:
- Only sets target if not congestion-throttled (isDeficientLocked)
- Does not set current layer (deferred to keyframe-based forwarder start)
- Logs at Debug level to avoid log noise
This prevents undefined layer state during manual subscription
with immediate quality upgrades (WithAutoSubscribe(false) +
SetVideoQuality(HIGH)).
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