Add native documents, compiled UI, and relocatable WebScene SDK

[danwalmsley]

Native applications can now use WebScene documents, events, styles, templates and Canvas directly from C++, compile HTML/CSS into application-owned C++ modules, and optionally link the same reusable JavaScript runtime. This provides the WebScene side of the AppScene SDK preview without rebuilding a runtime for each application.

• Add the native document/compiler foundation, typed and shared CSS integration, optional host adapters, and Kestrel compiler/interaction fixtures.
• Register compiled packages per engine with owned load snapshots, preserve shared-engine compatibility, and expose a synchronously detachable host work observer.
• Install independent WebScene::Core, NativeWeb, Runtime, WebGPU, and Compiler CMake targets, complete public header dependencies, application build/package helpers, and headless SDK consumers.
• Correct the interop ABI test to distinguish independently versioned hosting/file APIs, and await asynchronous visibility changes in the runtime tests.

Built on the now-merged WebGPU foundation #43. This branch also contains all commits from #60, including file services, startup viewport and SVG fixes, so #60 is subsumed by this PR.

Validation: 116 native compiler tests, 22 native document/compiler integration tests, 20 existing runtime/graphics tests, 18 TypeScript/tooling tests, 25 architecture/export tests, and repeated visibility contracts passed locally. The paired SDK built every standalone AppScene sample and both JavaScript configurations from a relocated, read-only installation with producer/source access denied. Independent hybrid builds ran concurrently; incremental asset/CSS/worker/module checks, runtime ownership tests, real Metal presentation/teardown and Kestrel smoke/parity passed. The final AppScene canvas resize fix was also verified interactively.

The binary SDK profile is a macOS ARM64 preview requiring macOS 26+ and LLVM 22.1.1. Additional desktop capabilities and a complete native C++ Kestrel remain outside this preview. A fresh full dependency rebuild and remote SDK workflow execution remain release qualification steps.