feat: support vtable generation

[nilslice]

Add VTable generation for runtime polymorphism

This PR adds automatic VTable generation to the Interface library, enabling runtime polymorphism through type erasure. Interfaces can now generate VTable-based wrappers that allow heterogeneous collections of implementations.

Example

// Define an interface
const AIProvider = Interface(.{
    .generate = fn ([]const u8) anyerror![]const u8,
    .embed = fn ([]const u8) anyerror![]f16,
    .query = fn ([]const u8) anyerror![][]const u8,
}, null);

// Create implementations (structs w/ interface methods defined above)
var openai = OpenAIMock.init(allocator);
var anthropic = AnthropicMock.init(allocator);

// Convert to VTable-based interface types
var providers = [_]AIProvider{
    AIProvider.from(&openai),      // Auto-generates VTable wrappers
    AIProvider.from(&anthropic),   // Different concrete type, same interface type
};

// NOTE: `.from()` is generated from `Interface({...})`, and provides the `.ptr` field on the output struct.

// demo generic function taking interface type arg
fn generate(provider: AIProvider, prompt: []const u8) ![]const u8 {
    return provider.vtable.generate(provider.ptr, prompt);
}

// Runtime polymorphism - call the implementation through the vtable
const response = try generate(providers[0].ptr, "prompt");