Two-phase HD: phase 1 keeps the fixed 1920x1080 iframe scaled down to latch
1080p; once the quality poll sees hd1080, switch to native responsive sizing so
YouTube renders its own controls (the gear especially) at full size instead of
scaled-down. A latched pick survives the shrink on a normal VOD.
CONDITIONAL / under observation: on one was_live stream in dev, quality dropped
hd1080 -> 480p ~2s after going native (YouTube re-caps to the small viewport);
held 1080p elsewhere. Gated behind HD_DROP_TO_NATIVE so phase 2 can be turned
off in one line to fall back to the guaranteed-1080p permanent-scale phase 1.
peter
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