refactor: delete two orphans, and record why the reparent design changed

`resolveVideoSource` chose between a local file and a remote URL for video
playback. Backend-owned stream selection took that decision into Rust —
`media_local_selection` for a downloaded file, `get_stream_selection` for a
streamed one — and its last caller went with it. What remained was the function
plus sixty lines of tests exercising nothing that ships.

`fittedVideoSize` computed the rendered size of a video letterboxed into its
container. Nothing has ever called it: it arrived with the fix that made the
video fill its viewport and was superseded by `object-fit: contain` in the same
change. There is some irony in a helper that models letterboxing sitting unused
beside a container that was not letterboxing at all — the bug fixed in the
previous commit was CSS, and this function would not have helped.

A survey for exported symbols referenced only by their own tests finds 22 more.
Most are legitimate — test mocks, deliberate reset hooks, public utility APIs —
and the rest are unrelated to this work, so they are left for a cleanup that can
be reviewed on its own terms rather than smuggled into a playback branch.

Also records in the spec why DR-231's design changed. Reparenting Tauri's
webview into a GtkOverlay aborts the process on the first click: Linux calls
`attach_resize_handler` unconditionally (the Windows path guards it with
`is_decorated()`), and its handler walks webview -> GtkBox -> GtkWindow with an
unwrap that an overlay breaks. So the webview is not moved at all — mpv draws
into the default vbox's own `draw` handler via `gdk_cairo_draw_from_gl()`, and
GTK's container-before-children order puts the webview on top for free. No
reparent, one less widget, and nothing a Tauri upgrade can invalidate by
assuming its own layout.

src/lib/components/player/videoFit.ts

@@ -15,32 +15,3 @@
 export function videoFitClass(): string {
   return "w-full h-full object-contain";
 }
-
-export interface FittedSize {
-  width: number;
-  height: number;
-}
-
-/**
- * The rendered size of a video of the given intrinsic dimensions once it has
- * been fitted into the container - i.e. scaled (up or down) so that it touches
- * the container on its constraining axis, with the other axis letter/pillar
- * boxed. Mirrors what `object-fit: contain` on a full-size element does.
- */
-export function fittedVideoSize(
-  intrinsicWidth: number,
-  intrinsicHeight: number,
-  containerWidth: number,
-  containerHeight: number,
-): FittedSize {
-  if (intrinsicWidth <= 0 || intrinsicHeight <= 0) {
-    return { width: 0, height: 0 };
-  }
-
-  const scale = Math.min(containerWidth / intrinsicWidth, containerHeight / intrinsicHeight);
-
-  return {
-    width: intrinsicWidth * scale,
-    height: intrinsicHeight * scale,
-  };
-}