build: include libmtmd in Apple XCFramework (opt-in LLAMA_BUILD_MTMD)

[theabecaster]

The Apple xcframework from build-xcframework.sh is built with LLAMA_BUILD_TOOLS=OFF, so no mtmd_* symbols end up in it. That means Swift / Obj-C consumers on iOS, macOS, tvOS, visionOS can't use vision even though the sources cross-compile fine.

This adds an opt-in LLAMA_BUILD_MTMD option (default OFF). When on and full tools are off, only the tools/mtmd subdirectory gets added so just the library target builds. The CLI exe targets in tools/mtmd/CMakeLists.txt are now gated on LLAMA_BUILD_TOOLS — without that, standalone visionOS xcodebuild fails with install TARGETS given no BUNDLE DESTINATION for MACOSX_BUNDLE executable target 'llama-mtmd-cli'.

build-xcframework.sh turns the option on and adds libmtmd.a to combine_static_libraries.

Headers aren't exposed via the framework module map. mtmd.h / mtmd-helper.h have C++ bits (method-bearing structs, struct mtmd_decoder_pos without the struct tag) that break pure-C clang module precompilation for Swift importers. Consumers declare the ABI via their own extern "C" shim — same workaround people use today.

Tested:

• build-xcframework.sh passes on macos-15 for all 8 platform slices
• nm ios-arm64/llama.framework/llama | grep -c ' _mtmd_' → 52
• Imports into an SPM iOS 17 app (Xcode 16.4, Swift 6 strict concurrency); runtime dlsym(RTLD_DEFAULT, "mtmd_init_from_file") is non-NULL on iPhone 15 Pro
• Legacy path (LLAMA_BUILD_MTMD=OFF or LLAMA_BUILD_TOOLS=ON) unchanged — full-tools build still pulls mtmd in via tools/

Disclosure: I used Claude to help draft the CMake wording. The design, debugging, and on-device verification are mine.

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[theabecaster]

Flagging the disclosure line at the bottom of the PR body / commit message — I used Claude for CMake wording, but the investigation and fix are mine.

For context on motivation: I'm building an iOS app that needs vision-capable on-device inference. Linking against the released xcframework failed because the mtmd_* symbols aren't in the binary; tracing it led back to LLAMA_BUILD_TOOLS=OFF in build-xcframework.sh. I worked around it locally with an extern "C" shim, but the root fix belongs upstream so other Apple-platform consumers don't hit the same wall. Happy to iterate on naming, defaults, or split the commit if preferred.