[FCE-2938] Re-enable simulcast variant switching

[sgfn]

Description

• Re-enables simulcast in webrtc-client
• Adds a new useSetTargetTrackEncoding hook in react-client
• Propagates the existing per-track encodingChanged event up to the client-level MessageEvents in ts-client
• Updates the minimal-react example to enable simulcast and render variant selection buttons

Motivation and Context

FCE-2938
Simulcast was deliberately disabled pending manual track selection support.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[linear]

FCE-2938 Re-enable simulcast

[Karolk99]

As discussed in the office, we don't need this event at the moment. Working example and setVideoQuality function instead of setTrackTargetVariant is enough for now. Also, you can add a test for switching layers and simulcast as another integration test.

[czerwiukk]

nitpick:

Suggested change

	const variantLabel = (variant: Variant | null | undefined): string => {
	switch (variant) {
	case Variant.VARIANT_LOW:
	return "Low";
	case Variant.VARIANT_MEDIUM:
	return "Medium";
	case Variant.VARIANT_HIGH:
	return "High";
	default:
	return "N/A";
	}
	};
	const variantLabelMap: Record<Variant, string> = {
	[Variant.VARIANT_LOW]: "Low",
	[Variant.VARIANT_MEDIUM]: "Medium",
	[Variant.VARIANT_HIGH]: "High",
	};

[czerwiukk]

suggestion: i feel this hook is a bit too "low level" as for the react client. instead, i'd suggest adding a setTarget method to useCamera, useMicrophone, useScreenShare, useCustomSource` instead of introducing a new hook.

[czerwiukk]

this will mitigate the need to pass over a track id. react client users don't even get to the "track" level, the hooks handle it for themselves. what do you think?

[sgfn]

the thing is, we're going to be using this function wrt remote tracks, not local ones. so, as far as I understand, we'd need to extend usePeers, and you'd still need to somehow specify the video track from a given peer you want to act upon...

ofc if you see a more elegant way, I'm all ears -- my React proficiency is nonexistent