Clamp lower-layer encoding against the top layer

[pblazej]

Summary

• Lower simulcast layers (q, h) are now clamped against the user-configured top encoding: maxFps unconditionally, and maxBitrate when scaleDownBy <= 1.0. A same-resolution lower layer can no longer outspend the top.
• Extracts computeVideoEncodings and a new pure helper computeSimulcastPresets into Sources/LiveKit/Support/Utils+VideoEncodings.swift so the preset-resolution logic is testable from LiveKitCoreTests (the WebRTC-typed return of computeVideoEncodings is not reachable via @testable import).

Fixes #1000.

Test plan

• xcodebuild test -scheme LiveKit -only-testing LiveKitCoreTests/SimulcastPresetsTests -destination 'platform=macOS' — 7 new tests pass.
• CI matrix (macOS 14/15/26 × Xcode 16.2/16.4/26.4, TSan job).
• Manual repro from Simulcast middle layer encoding parameters can exceed the user-configured top layer #1000: 1280×720 / 24 fps / 1.5 Mbps top with simulcastLayers: [.presetH720_169, .presetH360_169] no longer emits a 30 fps / 1.7 Mbps h layer.

[pblazej]

Note on the bitrate clamp — stricter than the JS / Android SDKs.

Both peer SDKs only clamp maxFps on lower layers; maxBitrate is left intact:

• client-sdk-js — encodingsFromPresets takes a sourceFramerate and applies Math.min(sourceFramerate, preset.encoding.maxFramerate) only: https://github.com/livekit/client-sdk-js/blob/595d4fbeb44b592d8b347dd141524f5f95e38fe6/src/room/participant/publishUtils.ts#L373-L381
• client-sdk-android — low/mid use preset.encoding.copy(maxFps = min(preset.encoding.maxFps, maxFps)) only: https://github.com/livekit/client-sdk-android/blob/71b321afbcdc1ecc5daf3ab1d9577e2a8f04c409/livekit-android-sdk/src/main/java/io/livekit/android/room/participant/LocalParticipant.kt#L871-L878

This PR additionally clamps maxBitrate when scaleDownBy <= 1.0 (i.e. the layer's resolution is not actually being scaled down vs. the top). That's the exact pathology from #1000 — presetH720_169 used as the h layer with a top encoding at the same 1280×720 resolution but lower bitrate. Layers that genuinely scale down still keep their preset bitrate, matching the JS/Android behavior.

Worth porting upstream into the Rust SDK too (livekit/src/room/options.rs compute_video_encodings has the same bug — no clamping at all).