tvOS support (appletvos + appletvsimulator)

I have tvOS working on top of v1.75.0 and would like to know whether you want it upstream before I open a PR.

Branch: https://github.com/tim4724/filament/tree/tvos-v1.75.0 — 6 commits, ~250 added lines across 15 files, no new backend. A work-in-progress game runs on an Apple TV 4K (A10X) at 3840x2160, 60 fps.

What it took: build.sh -p tvos plus toolchain and CMake plumbing (Metal forced on, since CocoaTouchGL isn't ported); availability guards and tvOS-prohibited API exclusions in MetalContext.mm, MetalHandles.mm and MetalState.mm; and backend_test building and running again on iOS/tvOS, behind INSTALL_BACKEND_TEST.

The branch also adds a presubmit build-tvos job. Per CONTRIBUTING.md that one is yours rather than mine, so I would drop .github/ from any PR.

Three things I would want your call on:

• Deployment target: the branch floors at tvOS 17.0.
• tvOS in the released XCFrameworks, or build-from-source only?
• backend_test on tvOS: opt-in, or wired into presubmit?

Happy to split it. The Metal availability fixes stand alone from the build work.

This issue and the branch were prepared with Claude Code.