fix(player): make native Android video opt-in again — it shipped as audio with no picture

DR-161 flipped experimentalNativeVideo on by default so picture-in-picture could
shrink a real video surface. On a device that shipped sound with a blank screen.

The decode path was never at fault. Logcat shows ExoPlayer running and feeding a
live SurfaceView with an active BufferQueue. The compositing was: the SurfaceView
sits behind the WebView, and the step that clears the opaque layers above it
never took effect — `WebView transparent = false` is logged, `= true` never
appears. The video was rendering correctly the whole time, behind an opaque page.

This is precisely the defect the flag existed to contain;
VideoPlayer.scrubRegression.test.ts had already recorded that "the native
SurfaceView has never been visible through the webview". Enabling it by default
shipped a verified decode path on top of an unverified display path.

Reverting costs nothing that matters: PiP does not depend on it — DR-160 drives
PiP from the WebView <video> — and working video outranks PiP showing a native
surface. The flag stays in Settings, now described as incomplete rather than as a
performance win, so anyone helping test it still can.

Fixing the compositing is the prerequisite for trying this default again (DR-172).

src/lib/components/player/VideoPlayer.scrubRegression.test.ts

@@ -23,12 +23,12 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 // ---- Mocks (must precede component import) --------------------------------
 
 const channelHandlers: Record<string, (event: any) => void> = {};
-// These tests pin the **flag-off** interim behaviour: when `experimentalNativeVideo`
-// is off, VideoPlayer overrides Android's native backend response to HTML5
-// rendering and stops the native backend. That flag now defaults to *on*
-// (DR-160, so picture-in-picture has a real surface to shrink into), so the
-// default no longer selects this path and the tests have to say which path they
-// are guarding rather than inherit it. (DR-161)
+// These tests pin the **flag-off** behaviour: when `experimentalNativeVideo` is
+// off, VideoPlayer overrides Android's native backend response to HTML5
+// rendering and stops the native backend. That is the default again (DR-172,
+// after native video shipped as audio with no picture), so this mock now agrees
+// with the default rather than opposing it — kept explicit so the tests state
+// which path they guard instead of inheriting whatever the default happens to be.
 vi.mock("$lib/stores/nativeVideo", async (importOriginal) => {
   const actual = await importOriginal<typeof import("$lib/stores/nativeVideo")>();
   return {