fix(macos): package whisper Metal dylibs with portable rpath

[fl0rianr]

Fixes the macOS Metal whisper-server release package so it includes the required .dylib dependencies and uses a portable @loader_path rpath.

Why

Downstream Lemonade CI fails before Metal initialization because dyld cannot load @rpath/libwhisper.1.dylib from the extracted macOS Metal artifact.

Changes

• Adds portable macOS rpath settings for the Metal build.
• Packages generated .dylib files alongside whisper-cli and whisper-server.
• Rewrites build-path dylib references to @rpath/<library>.
• Adds validation to fail packaging if whisper-server still references non-portable build paths or if libwhisper*.dylib is missing.

Test plan

• GitHub Actions macOS Metal build/package job should pass.
• Packaged whisper-server should no longer fail with Library not loaded: @rpath/libwhisper.1.dylib.