Fix macOS x86_64 cross-builds with bundled dependencies

[gregbenz]

Summary

• Propagate the standard macOS target settings to bundled CMake dependency builds.
• Add native Intel and Apple-Silicon-to-x86_64 macOS CI coverage, including artifact architecture checks and a native Intel CLI smoke test.
• Document thin x86_64 cross-compilation on Apple Silicon.

Motivation

CMAKE_OSX_ARCHITECTURES correctly targets libuhdr itself but was not forwarded to bundled dependencies. As a result, an Apple Silicon x86_64 cross-build using UHDR_BUILD_DEPS=1 could build host-architecture dependencies and fail at link time. Forwarding the existing macOS architecture, deployment-target, and sysroot settings keeps every target in the build on the requested architecture.

Validation

• Native arm64 Release build and CTest: 100% passed.
• Apple Silicon x86_64 cross-build with bundled dependencies and CTest under Rosetta: 100% passed.
• lipo confirmed the expected thin architecture for the library, CLI, tests, shared library, and bundled JPEG archive.
• The arm64 and x86_64 builds exported the same 43 public uhdr symbols.
• CLI encode, metadata probe, and decode passed on x86_64; arm64 successfully decoded the x86_64-produced JPEG and its decoded raw output was byte-identical.

Scope

• No new libraries or GitHub Actions are introduced.
• Universal arm64+x86_64 binaries remain out of scope; this supports one requested architecture per build.

Related: #383