Add bytecode chunk format and disassembler

[synapticvoid]

Summary

Add the core bytecode representation used by the compiler and VM, plus a readable disassembler for debugging.

Scope (MVP)

• Define OpCode enum for initial VM instructions
• Add chunk/code container:
  • instruction stream
  • constant pool
  • line/source mapping for diagnostics
• Add helpers to write instructions and constants
• Add debug disassembler that prints instruction offsets and decoded operands

Out of Scope

• VM execution loop
• AST lowering/compiler pass
• Function call frames and closures

Acceptance Criteria

• A manually assembled chunk can be created in tests
• Disassembler prints readable output for opcodes with and without operands
• Constant references decode to their literal values in disassembly output