Cast v2 video does not render on mobile inside Worlds (works on LANDs and on Desktop)

### Platform

Android (mobile). Desktop (Unity/Explorer) works correctly in the same scenario.

### App Version

RC 1.12.1 — `v1.12.1.1520-69a8567-prod` (PR #2687)

### Device Information

Not specified (Android device used by QA)

### Issue Description

The DCL Cast v2 fixes shipped in #2625 (which closed #2624 — "Cast v2 streams are never subscribed, and there is no stream selection") do not appear to work end-to-end on mobile when the scene runs **inside a World**.

- In Genesis City LANDs (tested at `-133,-42`) the Cast v2 stream **renders correctly** on mobile.
- In a World (tested `Ryumi.dcl.eth`) the video screen stays **black on mobile**, while the **same stream at the same time renders fine on Desktop**, same realm/world.

The stream was published via Admin Tools → Video screens → Media source: **DCL Cast** (active DCL Cast room, presenting).

This was the exact end-to-end case flagged as **unverified** in the 1.12.1 RC QA checklist for #2625, so it needs a review of the Cast v2 subscription/stream-selection path with World realms.

Note from Mateo: this is **not a regression** — LiveKit video streams were also not working in 1.12.0 — so it does not block the 1.12.1 RC. It may also turn out to be a scene / multiplayer-server issue where the stream is not being synced in Worlds, so the investigation should cover both the client subscription path and the World realm/comms setup.

### Expected Behavior

A DCL Cast v2 stream that renders on Desktop should also render on mobile when the scene is deployed to a World, with audio in sync — same as it does in Genesis City LANDs.

### Steps to Reproduce

1. Deploy/enter a scene with a `livekit-video://` screen in a World (e.g. `Ryumi.dcl.eth`).
2. As scene admin, open Admin Tools → Video screens → Media source → **DCL Cast**, activate the room and start streaming from the browser (camera or screenshare).
3. Open the same World on Desktop → the screen renders the video.
4. Open the same World on mobile (godot-explorer RC 1.12.1) → the screen stays black.
5. Repeat the same flow on a Genesis City LAND (e.g. `-133,-42`) → mobile renders the video correctly.

### Occurrence

100% in Worlds; 0% in Genesis City LANDs (works there).

### Investigation Notes / Scope

- Review the Cast v2 LiveKit track subscription and Unity-parity stream selection introduced in #2625 for World realms specifically.
- Check whether the video track is published/announced differently in a World (realm/comms room per World vs Genesis City island).
- Rule out a scene-side / sdk-multiplayer-server sync issue before changing client code.

### Related

- #2624 — LiveKit video transmission not working: Cast v2 streams are never subscribed (closed by #2625)
- #2625 — fix: subscribe Cast v2 LiveKit video tracks and port Unity stream selection
- #2687 — RFR Release Candidate 1.12.1 (where this was reported)

Reported by Ludmila Fantaniella during 1.12.1 RC QA.

Requested by Mateo via Slack