Build SwiftPM default Metal library resource

[RNT56]

Summary

• add a SwiftPM build-tool plugin that compiles the default Metal library into a package resource bundle
• wire the MLX target to use the generated metallib resource during package builds
• use URL-based PackagePlugin APIs and platform-aware Metal SDK/deployment flags

Draft status

This PR should stay draft while the runtime-loading path is reworked around the explicit metallib path API preferred upstream.

The previous automatic MLX bundle lookup dependency was closed/superseded. The intended route is now:

• Add metal::set_metallib_path() to override the metallib search path mlx#3597: C++ set_metallib_path API, merged
• Add mlx_metal_set_metallib_path() C API mlx-c#117: C wrapper, still draft/open
• Add GPU.setMetallibPath() for plugin bundle support #416: Swift GPU.setMetallibPath, still draft/open

I prepared a local proof branch at RNT56/mlx-swift:prep/swiftpm-metallib-via-path-api that combines this SwiftPM resource packaging with the explicit path API route. That proof branch finds mlx-swift_Cmlx.bundle/default.metallib, calls GPU.setMetallibPath(...) before the first GPU stream is created, and passes the runtime-loading smoke tests.

This PR should not be marked ready until the API chain above is available through upstream-owned commits/submodule pins. Once that is true, this branch can be rebuilt as a clean final PR containing only the resource packaging plus the SwiftPM resource-path hook and tests.

Validation so far

Current PR branch:

• swift build --target MLX
• bash -n tools/build-swiftpm-metallib.sh
• SDK_NAME=macosx tools/build-swiftpm-metallib.sh /tmp/mlx-swift-default-macos.metallib
• SDK_NAME=iphoneos tools/build-swiftpm-metallib.sh /tmp/mlx-swift-default-ios.metallib
• SDK_NAME=xros tools/build-swiftpm-metallib.sh /tmp/mlx-swift-default-xros.metallib
• git diff --check

Local proof branch:

• swift build --target MLX
• swift test --filter SwiftPMMetallibResourceTests
• swift test --filter StreamTests/testDeviceType
• git diff --check