feat(window): remember a side window's size across toggles

The F10 music playlist opened at a fixed fraction every time, so any manual resize was lost the moment it was toggled closed. Now the toggle captures the window's size on close and reopens at that height, for the rest of the session.

The mechanism is generic, not music-specific. cj/window-size-fraction (geometry-lib) is the pure kernel: a clamped window/frame ratio. cj/side-window-capture-size and cj/side-window-display (toggle-lib) wrap it for any display-buffer-in-side-window consumer — height for top/bottom, width for left/right — storing the remembered fraction in a caller-supplied state var. It mirrors the direction-split toggle pattern the vterm dispatchers already use, but for atomic side windows that can't be split.

music-config wires F10 to it: cj/music-playlist-window-height is the default, cj/--music-playlist-height holds the remembered value (in-memory, resets each session).

12 new tests across the two libs, Normal/Boundary/Error each covered.

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