061 reject zig struct syntax

// Test: Parser should reject Zig-style struct syntax in subflow implementations
// Zig uses .{ .field = value } but Koru uses branch_name { field: value }

const std = @import("std");

~[abstract] event fetch { url: []const u8 }
| success []const u8
| error i32

// WRONG: This is Zig syntax, not Koru
// Should be: ~input:fetch = success { body: "mock response" }
~input:fetch =
    .{ .success = "mock response" }