The SET_PROPERTY_FAILED error surfaced to Dart carries only mpv's own text
("unsupported format for accessing property"), which names the failure
mode, never the property. Every report of a refused write is therefore a
guessing game. Include name=value in the error description on all three
setProperty branches (generic, hdr-enabled, hdr-tone-mapping) and warn with
the same pair from MpvPlayer::SetPropertyAsync so the HDR transaction's
target-* writes — which never reach the channel — are attributable too.
mpv_request_log_messages is a single global level, and the vaapi probe and
"Using software decoding" fallback are MSGL_INFO, so at "warn" a silently
software-decoding session leaves no trace in the app log. Raise the request
level to "info" and observe hwdec-current natively, logging every decode
path transition from the player instead of relying on an overlay readout.
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