P2: Rebuild Sidecar around real OBS scene/look control

[iamfatness]

Problem

Sidecar currently feels disconnected: looks/previews do not reliably match OBS, source creation has been brittle, and operators cannot trust it as a production console.

Outcomes

• Sidecar controls actual OBS scenes through obs-websocket, not a standalone preview model.
• Creating/applying a look creates or updates the corresponding OBS scene, nested participant scenes, overlays, and source ordering.
• Preview is either WYSIWYG from OBS state or explicitly labeled as design-only.

Acceptance criteria

• Applying any built-in look changes the active OBS scene visibly.
• OBS scene audit reports exact missing/stale layers and offers repair.
• Sidecar never claims a look is live unless OBS state matches the render plan.

[iamfatness]

Progress on Sidecar OBS scene/look control:

• OBS websocket preview-scene failures now include an operator-facing explanation that SetCurrentPreviewScene requires OBS Studio Mode.
• The Look scene is still rendered into OBS; the error now points the operator to enable Studio Mode for PVW switching or use TAKE/program rendering.

Validation:

• cmake --build build-vs-release --config Release --target CoreVideoSidecar --parallel
• ctest --test-dir build-vs-release -C Release --output-on-failure passed 8/8

Committed in e58a14a.