Every other key in the MPRIS metadata map can be omitted safely,
because a client reading it renders a track with no title as a track
with no title. Art is different: KDE's applet treats an absent
mpris:artUrl as no news about the art and keeps drawing whatever the
last track had, so playing something without a cover left the previous
album's sleeve on screen — which reads as the wrong track playing
rather than as missing artwork.
The map's construction moves out of UpdateMetadata into a pure
metadataMap so it can be asserted on at all: everything else in this
file needs a live session bus, which is the same reason the Android
contract lives in an untagged file.
Closes#41
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>