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video fill its viewport and was superseded by `object-fit: contain` in the same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/player/localSource.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { downloadedFilePath, resolveVideoSource } from "./localSource";
+import { downloadedFilePath } from "./localSource";
 
 describe("downloadedFilePath", () => {
   // The download worker rewrites `downloads.file_path` to the absolute path it
@@ -29,74 +29,3 @@ describe("downloadedFilePath", () => {
     expect(downloadedFilePath("C:\\Users\\u\\AppData\\jellytau", stored)).toBe(stored);
   });
 });
-
-// A stand-in for Tauri's convertFileSrc, so the module stays pure.
-const toAssetUrl = (p: string) => `asset://localhost/${encodeURIComponent(p)}`;
-
-describe("resolveVideoSource", () => {
-  it("plays the downloaded file when one exists", () => {
-    const decision = resolveVideoSource({
-      localPath: "/home/u/.local/share/jellytau/movie.mp4",
-      remoteUrl: "https://server/Videos/abc/master.m3u8",
-      remoteNeedsTranscoding: true,
-      toAssetUrl,
-    });
-
-    expect(decision.isLocal).toBe(true);
-    expect(decision.url).toBe(toAssetUrl("/home/u/.local/share/jellytau/movie.mp4"));
-  });
-
-  it("never marks a local file as needing transcoding, even when the remote did", () => {
-    // The transcoded path re-requests a whole new stream URL on every seek.
-    // A local file seeks natively; sending it down that route would ask the
-    // server for a stream we deliberately avoided.
-    const decision = resolveVideoSource({
-      localPath: "/downloads/film.mkv",
-      remoteUrl: "https://server/Videos/abc/master.m3u8",
-      remoteNeedsTranscoding: true,
-      toAssetUrl,
-    });
-
-    expect(decision.needsTranscoding).toBe(false);
-  });
-
-  it("streams when nothing is downloaded, preserving the transcoding flag", () => {
-    const decision = resolveVideoSource({
-      localPath: null,
-      remoteUrl: "https://server/Videos/abc/master.m3u8",
-      remoteNeedsTranscoding: true,
-      toAssetUrl,
-    });
-
-    expect(decision).toEqual({
-      url: "https://server/Videos/abc/master.m3u8",
-      needsTranscoding: true,
-      isLocal: false,
-    });
-  });
-
-  it("streams a direct-play remote without claiming it transcodes", () => {
-    const decision = resolveVideoSource({
-      localPath: null,
-      remoteUrl: "https://server/Videos/abc/stream.mp4",
-      remoteNeedsTranscoding: false,
-      toAssetUrl,
-    });
-
-    expect(decision.needsTranscoding).toBe(false);
-    expect(decision.isLocal).toBe(false);
-  });
-
-  it("falls back to streaming for a blank path rather than building a dead asset URL", () => {
-    for (const localPath of ["", "   "]) {
-      const decision = resolveVideoSource({
-        localPath,
-        remoteUrl: "https://server/stream",
-        remoteNeedsTranscoding: false,
-        toAssetUrl,
-      });
-      expect(decision.isLocal).toBe(false);
-      expect(decision.url).toBe("https://server/stream");
-    }
-  });
-});