Add enum variant constructors to bytecode VM

[synapticvoid]

Summary

Compile enum declarations into variant constructors and support qualified access (e.g. Color.Red).

Checklist

Compiler

• Compile enum_declaration statement: for each variant, emit a struct_constructor callable with the qualified name (EnumName.VariantName)
• Track known_enum_names so zero-field variants referencing an existing enum get nested coercion (single-field wrapper)
• Compile qualified variant access (Color.Red) via field_access on a variable — resolve to the constructor in the environment

VM

• Ensure variant instances are regular StructInstance values with kind = .case
• Variant construction with fields reuses the struct construction path

Tests

• Simple enum declaration + variant construction (zero fields)
• Enum variant with fields (e.g. Shape.Circle(radius))
• Qualified access Enum.Variant resolves correctly
• Nested enum coercion (variant name matches an existing enum type)