Add builtin function call convention to bytecode VM

[synapticvoid]

Summary

Bridge existing builtin functions (print, toString, etc.) into the VM call convention so they can be invoked like user functions.

Checklist

Value representation

• Add BuiltinFn (or Function with builtin/user variants) to the VM Value type
• Register builtins into the VM globals at startup (mirror builtins.registerAll)

Compiler

• Resolve builtin names as globals (not locals) during compilation
• Compile builtin calls through the same fn_call path — emit args + op_call

VM

• In the op_call handler, branch on callable type: user fn → push call frame, builtin fn → invoke directly with evaluated args
• Pass RuntimeContext to builtins (needed for stdout/stderr access)
• Handle return value from builtins (push onto stack)

toString coercion

• Before passing struct instances to builtins, check for a qualified TypeName.toString method and call it (mirrors interpreter behavior in evaluateFnCall)

Tests

• print(42) produces correct output via VM
• print("hello") works
• Builtin with return value (e.g. toString) pushes result onto stack
• Struct instance passed to print triggers toString coercion

Depends on #43 (call frames) being merged first.