Add minimal VM execution loop for core opcodes

[synapticvoid]

Summary

Implement the first VM runtime loop capable of executing core stack-based bytecode.

Scope (MVP)

• VM state:
  • value stack
  • instruction pointer
  • globals table
• Execute initial opcodes:
  • constants/literals
  • local/global get/set/define (minimal form)
  • arithmetic/comparison
  • jump and jump-if-false
  • return
• Basic runtime error reporting (invalid opcode, stack underflow, type mismatch)

Out of Scope

• AST compiler/lowering
• User function call frames beyond minimal return path
• Struct/enum constructors and method dispatch
• try/catch error propagation semantics

Acceptance Criteria

• Hand-authored bytecode programs evaluate correctly (arithmetic, branches, variables)
• VM reports clear runtime errors for invalid programs
• Basic tests cover happy path + at least one runtime error case