Add metadata-only VRM loading

[noppefoxwolf]

Summary

• Add a metadata-only BinaryGLTF initializer that decodes the JSON chunk without retaining the BIN chunk
• Add VRMLoader.loadMeta APIs for URL and Data inputs
• Cover VRM0, VRM1, and URL metadata loading with tests

Tests

• swift test

[gemini-code-assist]

Code Review

This pull request introduces a new initializer init(jsonDataOnly:) in BinaryGLTF to parse only the JSON chunk of a GLTF binary, and adds helper methods in VRMLoader to load metadata (VRM0.Meta) from both VRM0 and VRM1 data or URLs. It also includes corresponding unit tests. The reviewer suggested refactoring BinaryGLTF to use a private initializer to reduce code duplication between the new and existing initializers.

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[gemini-code-assist]

There is significant code duplication between init(jsonDataOnly:) and the existing init(data:). We can refactor this by introducing a private initializer that handles both cases based on a boolean flag. This eliminates duplication, improves maintainability, and ensures that any future changes to the GLB parsing logic only need to be made in one place.

After applying this suggestion, you can also refactor the existing init(data:) to delegate to the private initializer:

public init(data: Data) throws {
    try self.init(data: data, jsonDataOnly: false)
}

    public init(jsonDataOnly data: Data) throws {
        try self.init(data: data, jsonDataOnly: true)
    }

    private init(data: Data, jsonDataOnly: Bool) throws {
        var offset = MemoryLayout<UInt32>.size // skip `magic`
        let rawVersion: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
        guard let version = GLTF.Version(rawValue: rawVersion), version == .two else {
            throw VRMError.notSupportedVersion(rawVersion)
        }
        self.version = version

        let length: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
        let chunk0Length: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
        let chunk0Type: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
        guard ChunkType(rawValue: chunk0Type) == .json else {
            throw VRMError.notSupportedChunkType(chunk0Type)
        }
        let jsonData = read(data, offset: &offset, size: Int(chunk0Length))
        let decoder = JSONDecoder()
        self.jsonData = try decoder.decode(GLTF.self, from: jsonData)

        if !jsonDataOnly, length > offset {
            let chunk1Length: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
            let chunk1Type: UInt32 = try read(data, offset: &offset, size: MemoryLayout<UInt32>.size)
            guard ChunkType(rawValue: chunk1Type) == .bin else {
                throw VRMError.notSupportedChunkType(chunk1Type)
            }
            binaryBuffer = read(data, offset: &offset, size: Int(chunk1Length)) as Data
        } else {
            binaryBuffer = nil
        }
    }