perry/media: Windows SystemMediaTransportControls lock-screen integration

[proggeramlug]

Follow-up to #351. The Windows backend (crates/perry-ui-windows/src/media_playback.rs v0.5.429) ships full playback via Windows.Media.Playback.MediaPlayer (WinRT) but setNowPlaying is a no-op — the volume HUD media tile, Bluetooth headphone media keys, and the Edge / Chromium Now Playing widget don't see anything from a Perry app.

Scope

WinRT MediaPlayer exposes a SystemMediaTransportControls property (and the related SystemMediaTransportControlsDisplayUpdater) that are the canonical surface. Wire setNowPlaying(handle, title, artist, album, artworkUrl) through it:

• Get the player's SystemMediaTransportControls via MediaPlayer.SystemMediaTransportControls.
• Set IsPlayEnabled / IsPauseEnabled / IsStopEnabled / IsNextEnabled / IsPreviousEnabled = true.
• Build a SystemMediaTransportControlsDisplayUpdater, set Type = MusicProperties.Music, populate MusicProperties.Title / Artist / AlbumArtist.
• For artwork: RandomAccessStreamReference.CreateFromUri(uri) on https:// URLs, or CreateFromFile on file:// paths — assigned to Thumbnail.
• Subscribe to the ButtonPressed event so headphone / volume-HUD play/pause/skip events route to the existing play / pause / stop FFI thunks.
• Push a PlaybackStatus update on every state-poller flip so the system UI doesn't show stale state.

windows-crate features

Already on Media_Playback + Media_Core + Foundation (added in v0.5.429). Will need Media (for SMTC types) and Storage_Streams (for RandomAccessStreamReference).

Acceptance

• setNowPlaying populates the Windows volume-HUD media tile.
• Headphone play/pause/skip buttons trigger the registered Perry callbacks via ButtonPressed.
• Edge / Chromium Now Playing widget shows the metadata.
• State poller pushes PlaybackStatus updates.

Files

• crates/perry-ui-windows/src/media_playback.rs::set_now_playing — currently a no-op
• crates/perry-ui-windows/Cargo.toml — add Media + Storage_Streams features