Waveform: stop the cursor immediately on every pause path

[niksedk]

Fixes #12233

The #12033 follow-up froze the interpolated waveform cursor the instant a pause is requested, but only for the ViewModel command paths (spacebar / Pause shortcut). Pausing via the video player's own toolbar button or a click on the video surface bypassed it: those paths call PlayOrPause() directly inside VideoPlayerControl, and the PlayPauseRequested event had no subscribers. The cursor then kept gliding until mpv's polled IsPlaying flipped ~100 ms later, and any estimate-vs-mpv drift showed up as a late slide after the settle window — "cursor keeps moving after pausing".

Changes

• PlayPauseRequested now carries intent (wasPlaying, captured before the toggle since mpv's IsPlaying lags the pause command) and is raised from the toolbar button, video-surface click and TogglePlayPause.
• InitVideoPlayer.MakeLayoutVideoPlayer wires it (docked + undocked players) to RequestPausePlayheadFreeze; the fullscreen player is wired at its creation site.
• The Stop button now freezes and pins the cursor to 0 immediately instead of lingering until mpv reports the new position.
• All pause-and-stay call sites in MainViewModel (dialog opening, subtitle single/double-click pause actions, waveform click pause actions, play-selection end) route through a PauseVideoAndFreezePlayhead helper. Transient pause-then-play paths (PlayNext etc.) keep pinning instead — a lingering freeze would hold the cursor for up to 300 ms after Play().
• Small reconciles after pause/stop (the ~100 ms mpv keeps playing after a pause request, or the stop-pin's 0.15 s arrival tolerance) are eased over ~160 ms instead of snapped, removing the tiny cursor jump that remained after the settle window. Real seeks beyond the 0.5 s resync threshold still snap; the playing branch is unchanged.

Verified manually on macOS: pausing via the player toolbar button, clicking the video surface, and the stop button now stop the cursor on the click, with no residual glide or late jump.

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