Add method dispatch and self injection to bytecode VM

[synapticvoid]

Summary

Support method calls on struct/enum instances with automatic self injection, including the special equals and toString methods.

Strategy: bound method value

Rather than a dedicated call_method opcode, use a .bound_method value variant. get_field resolves the method and bundles the receiver + function into a single typed value; call dispatches on it like any other callable. Keeps opcodes orthogonal, and bound methods become first-class values for free.

Checklist

VM — value

• Add .bound_method = { fn_idx, receiver } variant to vm/value.zig

VM — execution

• get_field fallback: if field not found as data, look up TypeName.fieldName in globals → push .bound_method
• call new arm: .bound_method → inject receiver as first arg, build frame

Compiler

• Compile struct method declarations: emit each method in struct.methods as TypeName.methodName in globals

Special methods

• equals: when op_equal encounters two struct instances, look up and call TypeName.equals if defined
• toString: coerce struct instances via TypeName.toString before passing to builtins (overlaps with Add builtin function call convention to bytecode VM #51)

Tests

• Method call on a struct instance
• self correctly refers to the receiver inside the method body
• Method with additional arguments beyond self
• Custom equals method used by == operator
• Custom toString method used by print
• Method on enum variant instance

Depends on #43 (call frames), #49 (struct instances), and #51 (builtins).

[synapticvoid]

Compiler method dispatch, equals dispatch, and tests implemented. toString deferred — tracked in new issue as part of the Executor/uniform calling convention refactor.